Comptamucc / Lab Experiments Fall 2015 Chem One

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Introduction to Chemistry Laboratory: A Lesson on Tools, Techniques and Measurements

PURPOSE: The purpose of this set of experiments (3 total) is to become familiar with the common types of laboratory glassware and equipment, and how to obtain and analyze data from these items. LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the following proficiencies:

1. Know which glassware (beakers, burettes, pipettes, graduated cylinders, flasks, etc) should be used in various circumstances.

2. Know how to “correctly” measure volume and mass (weight). 3. Become familiar with significant figures and its relationship to measurements and data recording (significant

figures). 4. Become familiar with the errors, precision and accuracy associated the various measurement tools and techniques. 5. Determine the density of liquids and solids. 6. Determine the best-fit straight line as a method to examine linear relationships and to use this relationship as a

predicative model such as in the determination of the percent copper and zinc in pennies based on density measurements.

7. Record laboratory data and observations. MATERIALS:

x Erlenmeyer Flasks o 125 mL o 250 mL

x beakers o 100 mL

x graduated cylinders o 10 mL o 25mL

x Burette o 50 mL

x Volumetric pipettes o 10 mL

x Measuring pipet o 10 mL

x Burette clamp and stand x Various liquids and solids for density determination measurements

o Liquids � Distilled water � Heptane � Carbon Tetrachloride

o Solids � Pennies � Copper � Zinc � Lead � Aluminum

BACKGROUND: Laboratory glassware. There are two major categories of laboratory glassware:

(1) those that contain a certain volume (volumetric flasks) and (2) those that deliver a certain volume (pipets, burets, and graduated cylinders).

“To Contain” glassware (sometimes labeled TC) is typically used for preparing solutions of known volume. “To Deliver” glassware (sometimes labeled TD) is used to transfer known volumes between containers.

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Some glassware is very carefully designed and marked for high accuracy/precision work (burets, pipets, and volumetric flasks), while other glassware is not intended for such work (beakers, Erlenmeyer flasks, and graduated cylinders). In high accuracy/precision work, the glassware must be clean. Not only does clean glassware avoid unwanted chemical contamination, but it also assures that delivered volumes of liquids will be correct. A dirty spot on the inside wall of a burette or pipet, for example, even if the spot itself does not occupy a significant volume, can cause a droplet of water to adhere to the wall, causing an error in the recorded volume of delivered liquid (less volume delivered). Measuring and recording data. The generally accepted rule for measuring volumes is to estimate one more digit beyond the digit associated with the closest spaced markings. For measurements obtained from devices which provide digital output, such as electronic mass balances or a Spectronic 20, the digits should all be written, including any trailing zeroes, with the understanding that the last digit is within “one” unit of the correct value. Errors (Uncertainties) in Measurements

Every measurement has a degree of uncertainty associated with it. The uncertainty derives from the measuring device and from the skill of the person doing the measuring. Let's use a volume measurement as an example. Say you are in a chemistry lab and need 7 mL of water. You could take an unmarked coffee cup and add water until you think you have about 7 milliliters. In this case, the majority of the measurement error is associated with the skill of the person doing the measuring. You could use a beaker, marked in 5 mL increments. With the beaker, you could easily obtain a volume between 5 and 10 mL, probably close to 7 mL, give or take 1 mL. If you used a pipette marked to with 0.1 mL, you could get a volume between 6.99 and 7.01 mL pretty reliably. It would be untrue to report that you measured 7.000 mL using any of these devices, because you didn't measure the volume to the nearest microliter. You would report your measurement using significant figures. These include all of the digits you know for certain plus the last digit, which contains some uncertainty.

Significant Figure Rules

x Non-zero digits are always significant. x All zeros between other significant digits are significant. x The number of significant figures is determined starting with the leftmost non-zero digit. The leftmost non-

zero digit is sometimes called the most significant digit or the most significant figure. For example, in the number 0.004205 the '4' is the most significant figure. The left hand '0's are not significant. The zero between the '2' and the '5' is significant.

x The rightmost digit of a decimal number is the least significant digit or least significant figure. Another way to look at the least significant figure is to consider it to be the rightmost digit when the number is written in scientific notation. Least significant figures are still significant! In the number 0.004205 (which may be written as 4.205 x 10-3), the '5' is the least significant figure. In the number 43.120 (which may be written as 4.3210 x 101), the '0' is the least significant figure.

x If no decimal point is present, the rightmost non-zero digit is the least significant figure. In the number 5800, the least significant figure is '8'.

Scientific notation

x Scientific notation is the way that scientists easily handle very large numbers or very small numbers. For example, instead of writing 0.0000000056, we write 5.6 x 10-9. So, how does this work?

x We can think of 5.6 x 10-9 as the product of two numbers: 5.6 (the digit term) and 10-9 (the exponential term).

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Conclusion: The number of significant figures is directly linked to a measurement. If a person needed only a rough estimate of volume, the beaker volume is satisfactory (2 significant figures), otherwise one should use the graduated cylinder (3 significant figures) or better yet, the buret (4 significant figures).

So, does the concept of significant figures deal with precision or accuracy? Hopefully, you can see that it really deals with precision only. Consider measuring the length of a metal rod several times with a ruler. You will get essentially the same measurement over and over again with a small reading error equal to about 1/10 of the smallest division on the ruler. You have determined the length with high precision. However, you don't know if the ruler was accurate to begin with. Perhaps it was a plastic ruler left in the hot Texas sun and was stretched. You don't know the accuracy of your measuring device unless you calibrate it, i.e. compare it against a ruler you knew was accurate. Note: in the laboratory, a good analytical chemist always calibrates her volumetric glassware before using it by weighing a known volume of liquid dispensed from the glassware. By dividing the mass of the liquid by its density, she can determine the actual volume and hence the accuracy of the glassware.

Accuracy Calculations (Absolute and Percent Error)

If you are comparing your value to an accepted value, you first subtract the two values so that the difference you get is a positive number. This is called taking the absolute value of the difference. Then you divide this result (the difference) by the accepted value to get a fraction, and then multiply by 100 to obtain the percent error.

So, Absolute error = |your result – accepted value|

Percent error = | your result - accepted value | x 100 accepted value

Several points should be noted when using this equation to obtain a percent error.

1) When you subtract, note how many significant figures remain after the subtraction, and express your final answer to no more than that number of digits.

2) If neither of the two values being compared is an "accepted value", then use either number in the denominator to get the fraction. If one value is more reliable than the other, choose it for the denominator.

3) Treat the % symbol as a unit. The fraction is dimensionless because units in the values will cancel.

4) Notice that the error is a positive number if the experimental value is too high, and is a negative number if the experimental value is too low.

Precision Calculations Frequently in science, an accepted or true value is not known. The accuracy of a measurement cannot be reported if an accepted value is unavailable. Precision is a measure of how reproducible experimental measurements are. Precision is reported as Deviation or Difference of values.

The Absolute Deviation, or Absolute Difference, of each measurement is the difference of each measurement from the mean or average:

Absolute deviation = |Measured value - Mean|

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The Percent Deviation, or Percent Difference, is the absolute deviation reported as a percentage:

Percent Deviation = Absolute Deviation x 100 Mean

For example, say that I ask people to guess my age and they guess 51, 49, 52, 52, and 51. How precise are their guesses?

Find average (51 + 49 + 52 + 52 + 51) / 5 = 51

For each measurement, find absolute deviation (difference from average)

|51-51|=0 |51-49|=2 |51-52|=1 |51-52|=1 |51-51|=0

Find average of absolute deviations

(0+2+1+1+0) / 5 = 0.8 Report using ± symbol (51±0.8)

Uncertainty in Calculations

Measured quantities are often used in calculations. The precision of the calculation is limited by the precision of the measurements on which it is based.

x Addition and Subtraction When measured quantities are used in addition or subtraction, the uncertainty is determined by the absolute uncertainty in the least precise measurement (not by the number of significant figures). Sometimes this is considered to be the number of digits after the decimal point.

Example: 32.01 m 5.325 m 12 m Added together, you will get 49.335 m, but the sum should be reported as '49' meters because the least precise measurement was 12 m. So that reading has an uncertainty of ±1 meter. In other words the measurement could be anywhere from 11.500….. to 12.499….. meters.

x Multiplication and Division When experimental quantities are multiplied or divided, the number of significant figures in the result is the same as that in the quantity with the smallest number of significant figures. If, for example, a density calculation is made in which 25.624 grams is divided by 25 mL, the density should be reported as 1.0 g/mL, not as 1.0000 g/mL or 1.000 g/mL.

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Losing Significant Figures Sometimes significant figures are 'lost' while performing calculations. For example, if you find the mass of a beaker to be 53.110 g, add water to the beaker and find the mass of the beaker plus water to be 53.987 g, the mass of the water is 53.987-53.110 g = 0.877 g. The final value only has three significant figures, even though each mass measurement contained 5 significant figures. Rounding and Truncating Numbers There are different methods which may be used to round numbers. The usual method is to round numbers with digits less than 5 down and numbers with digits greater than 5 up (some people round exactly 5 up and some round it down). Example: If you are subtracting 7.799 g - 6.25 g your calculation would yield 1.549 g. This number would be rounded to 1.55 g, because the digit '9' is greater than '5'.

Exact and Inexact Numbers

There are two kinds of numbers in the world: x exact:

o example: There are exactly 12 eggs in a dozen. o example: Most people have exactly 10 fingers and 10 toes.

x inexact numbers: o example: any measurement.

If I quickly measure the width of a piece of notebook paper, I might get 220 mm (2 significant figures). If I am more precise, I might get 216 mm (3 significant figures). An even more precise measurement would be 215.6 mm (4 significant figures).

Exact Numbers Sometimes numbers used in a calculation are exact rather than approximate. This is true when using defined quantities, including many conversion factors, and when using pure numbers. Pure or defined numbers do not affect the accuracy of a calculation. You may think of them as having an infinite number of significant figures. Pure numbers are easy to spot because they have no units. Defined values or conversion factors, like measured values, may have units. Practice identifying them! Example: You want to calculate the average height of three plants and measure the following heights: 30.1 cm, 25.2 cm, 31.3 cm; with an average height of (30.1 + 25.2 + 31.3)/3 = 86.6/3 = 28.87 = 28.9 cm. There are three significant figures in the heights. Even though you are dividing the sum by a single digit, the three significant figures should be retained in the calculation.

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Introduction to Chemistry Laboratory A Lesson on Tools, Techniques and Measurements

Experiment 1

PRECISION AND ACCURACY OF SCIENTIFIC GLASSWARE

Learning Objectives for Experiment 1

1. Learn which glassware (beakers, burettes, pipettes, graduated cylinders, flasks, etc.) should be used in

various circumstances. 2. Learn how to measure volume and mass (weight). 3. Become familiar with significant figures and its relationship to measurements and data recording. 4. Become familiar with the errors, precision and accuracy associated the various measurement tools and

techniques. 5. Learn how to record laboratory data and observations.

In this part of the experiment you will develop and employ procedures utilizing water, which has a density of 1 g/ml, to determine how precisely and accurately volumes can be delivered with the following glassware. Be sure to take into account if the glassware is designed to deliver (TD) or contain (TC) a certain volume. Also, repeat measurements at least three times in order to determine precision. After collecting all of the measurements you will determine the precision and accuracy in both absolute and relative terms for each of the different types and sizes of glassware. You will then arrange this information in a table and discuss the results of your findings. Note that a table for entering the data is included in this document. However, other tables or figures should be utilized in your lab report in order to be able to compare precision and accuracy of various glassware. It is VERY important to note that the purpose of this and any experiment is not only to collect the data but also to interpret and then discuss the results. Be sure that all data collected as well as all calculations include the correct number of significant figures and be sure to include sample calculations for all reported results. Remember, a major part of this exercise is learning about significant figures and their relevance to precision and accuracy. To Contain Glassware (TC)

x Erlenmeyer Flasks o 125 mL (measure 100mL) o 250 mL (measure 150 mL)

x Beakers o 100 mL (measure 50 mL)

x Graduated cylinders o 10 mL (measure 7.5 ml) o 25 mL (measure 22 mL)

To Deliver Glassware (TD) x Burette

o 50 mL (measure 35.00 mL) x Volumetric pipettes (measure the volume the pipettes are designed for)

o 10 mL x Measuring pipet

o 10 mL (measure 7.5 mL)

16

mass of mass of flask mass of water volume of waterTrial empty flask (g) with water (g) in flask (g) in flask (mL) Absolute error % error Absolute deviation % deviation

123

mass of mass of flask mass of water volume of water Accuracy PrecisionTrial empty flask (g) with water (g) in flask (g) in flask (mL) Absolute error % error Absolute deviation % deviation

123


123


123


123


123


123


123

Determination of Accuracy and Precision of 50 mL Burette for Measurement of 35 mL of Solution

Accuracy Precision

Determination of Accuracy and Precision of 10 mL Graduated Cylinder for Measurement of 7.5 mL of Solution

Accuracy Precision

Determination of Accuracy and Precision of 25 mL Graduated Cylinder for Measurement of 22 mL of Solution

Accuracy Precision

Determination of Accuracy and Precision of 10 mL Measuring Pipette for Measurement of 7.5 mL of Solution

Accuracy PrecisionDetermination of Accuracy and Precision of 125 mL Erlenemeyer Flask for Measurement of 100 mL of Solution

Determination of Accuracy and Precision of 100 mL Beaker for Measurement of 50 mL of Solution

Accuracy Precision

Determination of Accuracy and Precision of 250 mL Erlenemeyer Flask for Measurement of 150 mL of Solution

Accuracy Precision

Determination of Accuracy and Precision of 10 mL Volumetric Pipette for Measurement of Volume it was Designed for

Accuracy Precision

Data Sheet, Experiment I (Week 2)

Student Name: Partner(s) Name(s)

Date: Lab Section:

17


Experiment 1 PRECISION AND ACCURACY OF SCIENTIFIC GLASSWARE

Postlaboratory (Lab Report) Assignment Write a lab report on the Precision and Accuracy of Scientific Glassware experiment. The lab report will only

consist of the Result section. Follow the lab report guidelines from the syllabus (Appendix I, p. 10) and the

grading rubric of this experiment (next page, this document). Please note that turning in only the spreadsheet

data as a lab report will be considered an incomplete assignment.

18

Precision and Accuracy of Scientific Glassware Lab Report Experiment 1 Grading Rubric (100 points)

The lab report (100 points): (1) Presents table(s) clearly and accurately. In other words, it is typed and easy to read. (16 points). (2) Shows acceptable errors (5-10%) of accuracy and precision values (40 points, 5 points each table). (3) Includes statements of all eight experiments (24 points). (4) Contents statements of trends in accuracy when using different glassware (flasks, beakers, pipettes, etc.). It should also state which glassware is the most/least accurate (20 points)

19


Experiment 2

DETERMINING THE DENSITY OF VARIOUS LIQUIDS AND SOLIDS


1. Learn how to determine the density of liquids and solids. 2. Become familiar with significant figures and its relationship to measurements and data recording

(significant figures). 3. Become familiar with the errors, precision and accuracy associated the various measurement tools and

techniques. 4. Learn how to record laboratory data and observations.

In this part of the experiment you will develop and utilize procedures to determine the density of the following materials.

1. Determine the density of the following. o Liquids

� Water � Heptane � Carbon Tetrachloride

o Solids � Copper � Zinc � Lead � Aluminum

2. Compare the accuracy (absolute and % errors) of your calculated densities to accepted literature values. 3. Determine the precision (absolute and percent deviation) for each of your measurements and compare

your results compared to literature values.

Repeat all measurements at least three times in order to determine precision. After collecting all of the measurements, as mentioned above, you will determine the precision and accuracy in both absolute and relative terms. You will then arrange this information in a table and discuss the results of your findings. Be sure that all data collected as well as all calculations include the “correct” number of significant figures. Remember, a major part of this exercise is learning about significant figures and their relevance to precision and accuracy.

NOTE: Set up data sheets similar to the ones used in previous experiment in order to organize your data to be able to calculate precision and accuracy of your measurements.

20

Introduction to Chemistry Laboratory

A Lesson on Tools, Techniques and Measurements Experiment 2

DETERMINING THE DENSITY OF VARIOUS LIQUIDS AND SOLIDS

Prelaboratory Assignment Prepare a typed pre-lab report on the Determining the Density of Various Liquids and Solids to be turned in

at the beginning of the lab period. Read carefully the pre-lab guidelines in the Section VIII of the Syllabus. This

experiment should include:

1) Objectives and Background

2) Experimental Procedure

Materials

Steps to determine the density of liquids and solids

List of density values of Heptane, Carbon Tetrachloride, Cu, Zn, Pb, Al.

Postlaboratory (Lab Report) Assignment Write a lab report on the Determining the Density of Various Liquids and Solids experiment. The lab report

will consist of the Result and Discussion section. Follow the lab report guidelines from the syllabus (Appendix

I, p. 10) and the grading rubric of this experiment (next page, this document).

21

Determining the Density of Various Liquids and Solids Lab Report Experiment 2 Grading Rubric (100 points)

Pre-lab Report (25 points): Objectives and Background (10 points)

Experimental Procedure (15 points)

Materials

Steps to determine the density of liquids and solids

List of density values of Heptane, Carbon Tetrachloride, Cu, Zn, Pb, Al.

Lab report (75 points): Results (50 points) (1) Presents table(s) and graphs clearly and accurately. All data (pure fluid, fluid mixture,

unknown and solid samples) are in cluded. No hand drawn figures are accepted.

(20 points)

(2) Shows acceptable errors of accuracy and precision values. (10 points)

(3) Includes statements of the trends shown by your data (density data trends, any

relationships between data, precision and accuracy of your results, etc.). For example, did

an increase in a measured variable result in an increase/decrease in the corresponding

calculated variable? etc. (20 points)

Discussion (25 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). (15 points) Data: Are the data reasonable?

Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ... (10 points)

22


Experiment 3 DETERMINING THE PERCENT COPPER AND ZINC IN PENNIES FROM DENSITY


1. Develop a method to determine the percent copper and zinc in pennies from density measurements. 2. Learn how to use the best-fit straight line as a method to examine linear relationships and to use this

relationship as a predicative model to determine the percent copper and zinc in pennies based on density measurements.

3. Learn how to record laboratory data and observations.

Determine the density of the following: x Various percentages of zinc and copper mixtures

o 0% Cu, 100% Zn o ~10% Cu, ~90% Zn o ~20% Cu, ~80% Zn o ~30% Cu, ~70% Zn o ~40% Cu, ~60% Zn o ~50% Cu, ~50% Zn o ~60% Cu, ~40% Zn o ~70% Cu, ~30% Zn o ~80% Cu, ~20% Zn o ~90% Cu, ~10% Zn o 100% Cu, 0% Zn

x Pennies o using 5, 10, 20 and 30 pre1982 pennies o using 5, 10, 20 and 30 post1982 pennies

x Compare the precision and accuracy of your results to literature values for pre and post 1982 pennies when using various numbers of coins (5, 10, 20 and 30) in the determination of the density of the coins.

From the density data, develop a method to determine the percent copper and zinc in pre 1982 and post 1982 pennies. Compare your results (absolute and % errors) to literature values.

Below is an example of a linear relationship and how the best fit line can be utilized as a predictive model.

A group of students are driving from Los Angeles (L.A.) California to New York City and plan to travel nonstop and switch drivers as needed. The route they have chosen was estimated to be about 2,773 miles according Google maps. Every time they stopped for gas one of the students (Sally) recorded the distance traveled and the time since they left L.A. Once they reached Tulsa, Oklahoma one of students asked Sally how

NOTE: As was done in the previous experiments, set up data sheets in order to calculate the precision and accuracy of your measurements.

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24


Experiment 3

DETERMINING THE PERCENT COPPER AND ZINC IN PENNIES FROM DENSITY

Prelaboratory Assignment

Prepare a typed pre-lab report on the Determining the Percent Copper and Zinc in Pennies from Density to

be turned in at the beginning of the lab period. This experiment should include:

1) Objectives and Background

2) Experimental Procedure

Materials

Steps to determine the density of pennies

3) Graph method to determine the percent copper and zinc in pennies. The graph consists of theoretical

values of a Cu-Zn mixture in different proportions (y-axis) vs. % Cu (x-axis).

Postlaboratory (Lab Report) Assignment

Write a lab report on the Determining the Percent Copper and Zinc in Pennies from Density experiment.

The lab report will consist of the Experimental Procedure, Result and Discussion sections. Follow the lab

report guidelines from the syllabus (Appendix I) and the grading rubric of this experiment (next page, this

document).

25

Determining the Percent Copper and Zinc in Pennies from Density

Lab Report Experiment 3 Grading Rubric (100 points)

Pre-lab Report (25 points): Objectives and Background (5 points)

Experimental Procedure (10 points)

Materials

Steps to determine the density of pennies

Graph method to determine the percent copper and zinc in pennies. The graph consists of theoretical values of a

Cu-Zn mixture in different proportions (y-axis) vs. % Cu (x-axis). (10 points)

Lab report (75 points): Procedure and Observations in the Lab report (20 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional Results (35 points) (1) Present table(s) and graphs clearly and accurately. All data (density values of pennies, Cu- Zn mixtures, Cu and Zn percentage determinations with the corresponding graph) are included. No hand drawn figures are accepted. (20 points) (2) Show acceptable errors of accuracy and precision values. (5 points) (3) Content statements of the trends shown by your data (density data trends, any relationships between data, precision and accuracy of your results, etc.). For example, did an increase in a measured variable result in an increase/decrease in the corresponding calculated variable? etc. (10 points) Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). (10 points) Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed

improperly? ... (10 points)

26

Introduction to Chemistry Laboratory Experiment 4

BEHAVIOR OF GASES

The purpose of this investigation is to conduct a series of experiments to illustrates different gas laws. You will be given a list of equipment and materials and some general guidelines to help you get started with each experiment. Three properties of gases will be investigated: pressure, volume, temperature. By assembling the equipment, conducting the appropriate tests, and analyzing your data and observations, you will be able to describe the gas laws, both qualitatively and mathematically.

OBJECTIVES In this experiment, you will

x Conduct a set of experiments, each of which illustrates a gas law. x Gather data to identify the gas law described by each activity. x Complete the calculations necessary to evaluate the gas law in each activity. x From your results, derive a single mathematical relationship that relates pressure, volume, and

temperature. MATERIALS

LabQuest Three 600mL beakers LabQuest App rubber stopper assembly with two-way valve Vernier Gas Pressure Sensor hot plateTemperature Probe Ice 20 mL gas syringe 125 mL Erlenmeyer flask

PRE-LAB EXERCISE Review each part of this experiment before starting your work. You will need to decide the best way to conduct the testing, so it is wise to make some plans before you begin. You may wish to conduct a test run without collecting data, in order to observe how the experiment will proceed.

In each part of the experiment, you will investigate the relationship between two of the four possible variables, the other two being constant. In this pre-lab exercise, sketch a graph that describes your hypothesis as to the mathematical relationship between the two variables; e.g., direct relationship or inverse relationship.

Part I Pressure, P, and volume, V (temperature and number of molecules constant). Part II Pressure, P, and absolute temperature, T (volume and number of molecules constant).

PROCEDURE Part I Pressure and Volume

2. Position the piston of a plastic 20 mL syringe at 10 mL. Attach the syringe to the valve of the Gas Pressure Sensor, as shown in Figure 1. A gentle half turn should connect the syringe to the sensor securely. Note:

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29


BEHAVIOR OF GASES

Prelaboratory Assignment Prepare a typed pre-lab report on the Behavior of Gases experiment to be turned in at the beginning of the lab

period. This experiment should include:

(1) A summary of the concept(s). The Gas Laws: Pressure Volume Temperature Relationships

(Boyle’s Law, Charles’s Law and the Ideal Gas Law).

(2) Graphs accompanied by tables that show the expected results of the experiment(s).

Examples: plots of pressure vs. volume, pressure vs. temperature, etc. Please note that

references must be included for the graphs acquired via online/library/textbook

sources.

Postlaboratory (Lab Report) Assignment Write a lab report on the Behavior of Gases experiment. The lab report will consist of the Introduction,

Experimental Procedure, Result and Discussion sections. Follow the lab report guidelines from the syllabus

(Appendix I) and the grading rubric of this experiment (next page, this document).

30

Behavior of Gases Lab Report Experiment 4

Grading Rubric (100 points) PRE-LAB REPORT (25 points) (1) A summary of the concept(s). The Gas Laws: Pressure Volume Temperature Relationships

(Boyle’s Law, Charles’s Law and the Ideal Gas Law). (15 points)

(2) Graphs accompanied by tables that show the expected results of the experiment(s). Examples: plots of pressure vs. volume, pressure vs. temperature, etc. Please note that references must be included for the graphs acquired via online/library/textbook sources. (10 points) LAB REPORT (75 points) Introduction (15 points) Includes problem, background information, hypothesis and expected results. Experimental Procedure (10 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional Results (30 points) (1) Presents table(s) and calculations clearly and accurately. All data: Volume, Pressure, Temperature (tables and graphs), linear relationships are included. (2) Contents statements for each table/calculation and summarizes the overall trend shown by your data. Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ...

31


DETERMINATION OF ABSOLUTE ZERO

Temperature is a measure of the kinetic energy (or movement) of particles inside of an object. If particles have a lot of kinetic energy, the object will have a high temperature, and if particles have less kinetic energy, the object will have a low temperature. Thus, the lowest temperature an object can have happens when the particles in the object are not moving at all. This temperature represents absolute zero, it is zero Kelvin (the SI unit for temperature), and -273 C° on the Celsius scale. Experiment Goals

(1) Estimate the value of absolute zero. (2) Using the equipment provided below, design an experiment to estimate the value of

absolute zero and its error in degrees Celsius. (3) Describe both your design process and the experimental procedure. Show any

calculations necessary to achieve your goal. (4) The lab report should explain any propagation of error. It also should state any

assumptions regarding uncertainties, the major sources of uncertainty, and suggest experimental methods for increasing the accuracy of your estimate. Explaining how you fit curves to any data, how data errors were accommodated in that process, and how uncertainties were ascribed to curve-interpolated or extrapolated estimates are also part of the learning objectives.

MATERIALS Ethanol Ethylene glycol Dry ice (cooling agent) LabQuest Five 600mL beakers LabQuest App Rubber stopper assembly with two-way valve Vernier Gas Pressure Sensor Ice Temperature Probe 125 mL Erlenmeyer flask

32


DETERMINATION OF ABSOLUTE ZERO

Prelaboratory Assignment Prepare a typed pre-lab report on the Determination of absolute zero experiment to be turned in at the

beginning of the lab period. This experiment should include:

(1) Experimental procedure: how to set up cooling baths using mixtures (150 mL total volume)

of ethylene glycol and ethanol (e.g., 0% Ethanol, 25% Ethanol, 50% Ethanol, 100% Ethanol)

over dry ice to obtain different temperature and pressure values.

(2) Graph method (i.e., equation of a line) to estimate the absolute zero temperature.

Postlaboratory (Lab Report) Assignment Write a lab report on the Determination of absolute zero. The lab report will consist of the Introduction,

Experimental Procedure, Results, Discussion and Conclusion sections. Follow the lab report guidelines from

the syllabus (Appendix I) and the grading rubric of this experiment (next page, this document).

33

Determination of Absolute Zero Lab Report Experiment 5

Grading Rubric (100 points) PRE-LAB REPORT (25 points) (1) A summary of the concept(s). (10 points) (2) Experimental procedure: how to set up cooling baths using mixtures (150 mL volume) of ethylene glycol and ethanol in different proportions over dry ice to obtain different temperature and pressure values. Show calculations. (10 points) (3) Graph method (i.e., equation of a line) to estimate the absolute zero temperature. Include references. (5 points) LAB REPORT (75 points) Introduction (10 points) Includes problem, background information, hypothesis and expected results. Experimental Procedure (10 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional Results (25 points) (1) Presents table(s) and calculations clearly and accurately. All data: Pressure, Temperature (tables and graphs), linear relationship(s), the absolute zero temperature estimated value are included. (2) Contents statements for each table/calculation and summarizes the overall trend shown by your data. Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ... Conclusions (10 points) State what it was learnt about the scientific concept(s) of the lab from doing the experimental procedure; give enough details of what it was accomplished to be convincing. Include in your conclusions a section that summarizes all of your data and conclusions.

34

Introduction to Chemistry Laboratory

Experiment 6 AN INTRODUCTION TO TITRATION: STANDARDIZATION OF HCl and NaOH

Experimental Procedure

Standardization of ~0.10M HCI 1- Weigh duplicate ~0.15 g samples of previously dried standard Na2CO3. Dissolve samples in ~100- ml distilled water; if it does not dissolve quickly, you may warm the solution. 2 - Cool the solution to room temperature and add ~ 0.5 to 1 ml of bromocresol green indicator, solution turns into blue. Titrate it with HCl until green color is reached. (DO NOT OVER TITRATE) 3 - Heat and boil out CO2 gently. You should obtain a blue color again at the end of this step. Cool to room temperature, and continue titration until yellow color is reached. (DO NOT OVER TITRATE) 4 - Calculate the molarity of HCI, it should agree to about two parts per thousand. If they do not agree, repeat the procedure for the third time and take the avg. of the two closest results. Standardization of ~0.10M NaOH 1- Weigh duplicate samples of 0.50-0.60 g of previously dried standard potassium hydrogen phthalate (KHP) into 250- ml flasks. Dissolve each sample in ~100- ml boiled distilled water. 2 - Cool to room temperature, add 2-4 drops of phenolphthalein indicator. Titrate it with NaOH until the first appearance of a permanent, pink color. 3 - Back titrate with your standard HCl if you miss the end point. Calculate the molarity of NaOH solution. 4 - If runs do not agree to about two parts per thousand, repeat for the third time and take the avg. of the two closest results. DATA Standardization of HCI

Trial 1 Trial 2 Trial 3 wt. of standard Na2C03 _______g _______g _______g ml of HCI ________ ________ ________ molarity of HCI ________ ________ ________ avg. molarity of HCI ____________________

35

Standardization of NaOH

Trial 1 Trial 2 Trial 3 wt. of standard KHP _______g _______g _______g ml of NaOH ________ ________ ________ molarity of NaOH ________ ________ ________ avg. molarity of NaOH ____________________ Things to be done before the experiment Obtain the burette set. Wash it well with detergent water and then rinse it out couple of times with tap and distilled water. Rinse it out once more with the solution to be standardized and then fill it up with the same solution. Be sure that the tip of the burette is free of air bubbles. Caution: NaOH is very corrosive, do not spill and if you do, wash it off immediately with an excess of water.

36


AN INTRODUCTION TO TITRATION: STANDARDIZATION OF HCl and NaOH


Prepare a typed pre-lab report on the An Introduction to titration: Standardization of HCl and NaOH

experiment to be turned in at the beginning of the lab period. This experiment should include:

(1) Objectives & Background

(i.e., importance of standardizing solutions, what standards are used, acid - base reactions, titration

techniques, etc…)

(2) Chemical reactions (5 points)

a) HCl and Na2CO3

b) NaOH and KHP (HKC8H4O4)

(3) Number of moles of HCl and NaOH (show calculations) when 0.15 g of Na2CO3 and 0.50 g

of KHP are used

Postlaboratory (Lab Report) Assignment Write a lab report on the An Introduction to titration: Standardization of HCl and NaOH. The lab report will

consist of the Introduction, Experimental Procedure, Results, Discussion and Conclusion sections. Follow

the lab report guidelines from the syllabus (Appendix I) and the grading rubric of this experiment (next page,

this document).

37

An Introduction to titration: Standardization of HCl and NaOH Lab Report Experiment 6

Grading Rubric (100 points) PRE-LAB REPORT (25 points) (1) Objectives & Background (10 points)

(i.e., importance of standardizing solutions, what standards are used, acid - base reactions, titration techniques, etc…)

(2) Chemical reactions (5 points) a) HCl and Na2CO3 b) NaOH and KHP (HKC8H4O4) (3) Number of moles of HCl and NaOH (show calculations) when 0.15 g of Na2CO3 and 0.50 g

of KHP are used (10 points). LAB REPORT (75 points) Introduction (10 points) Includes problem, background information, hypothesis and expected results. Experimental Procedure (10 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional sources. Results (25 points) (1) Presents table(s) and calculations clearly and accurately. All data (grams of Na2CO3 and KHP samples, mL HCl, molarity calculations, etc..) are included. (2) Shows acceptable errors of precision measurements. (3) Contents statements for each table/graph/calculation and summarizes the overall trend shown by your data. Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ... Conclusion (10 points) State what it was learnt about the scientific concept(s) of the lab from doing the experimental procedure; give enough details of what it was accomplished to be convincing. Include in your conclusions a section that summarizes all of your data and conclusions.

38


INVESTIGATING STOICHIOMETRY WITH SODIUM SALTS OF CARBONIC ACID

The purpose of this lab is to investigate and better understand chemical stoichiometry. In this experiment, you will be reacting sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3) with hydrochloric acid (HCl) as shown below to produce sodium chloride, water and carbon dioxide. Below is the balanced chemical reaction of HCl with sodium bicarbonate. NaHCO3 + HCl = NaCl + H20 + CO2 reactants products At the beginning of the experiment, you will obtain the mass in grams of two “unknown” solids. One will be sodium bicarbonate and the other will be sodium carbonate. Before using the two conversion factors discussed above, you will be able to carry out the following conversion: grams NaHCO3 - moles NaHCO3 - moles NaCl - grams NaCl NOTE: The formulas and equation for NaHCO3 are given above. Before you begin the experiment you should write a balanced reaction for reaction of HCl with Na2CO3 as well as determine how much HCl would be needed to react completely with the “unknown” material. In other words, figure about how much HCl would be needed to react if the “unknown” was sodium carbonate and how much would be needed if the “unknown” was sodium bicarbonate. In addition, after completing the experiment calculate theoretical, actual yields for these reactions, and identify what each of the “unknowns” are. The grams of NaCl that you determine in this calculation is called the theoretical yield. At the end of the experiment, you will determine the mass in grams of the sodium chloride product and this is called the actual yield. According to the law of conservation of mass, the actual yield should be equal to the theoretical yield. However, due to human and experimental errors, it very seldom is. The percent yield is calculated using the following equation: Percent yield = (Actual yield/Theoretical yield) x 100 Development of Laboratory Procedures In order to develop procedures to determine the amount of NaCl formed in the reaction it is important to know that when sodium carbonate and sodium bicarbonate react with HCl the two products are NaCl and CO2. CO2 is given off as a gas thus the only things left in solution are NaCl and water. If the solution is heated and the water evaporated the only thing that should remain in the container would be the NaCl that was formed in the reaction, assuming you did not add too much HCl. Note that you will be developing procedures to determine what the unknown is as well as the yields using two different but complementary techniques. One will involve determining the mass of NaCl formed during the reaction of HCl with the “unknown” and the other will involve development of a titrimetric technique in which you will equate moles of HCl to the moles of reactant as was done in the standardization procedures.

39

ADDITIONAL BACKGROUND INFORMATION Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers. For example, in a reaction that forms ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3: N2 + 3H2 → 2NH3 Stoichiometry can be used to find quantities such as the amount of products (in mass, moles, volume, etc.) that can be produced with given reactants and percent yield (the percentage of the given reactant that is made into the product). Stoichiometry calculations can predict how elements and components diluted in a standard solution react in experimental conditions. Stoichiometry is founded on the law of conservation of mass: the mass of the reactants equals the mass of the products. Reaction stoichiometry describes the quantitative relationships among substances as they participate in chemical reactions. In the example above, reaction stoichiometry describes the 1:3:2 ratio of molecules of nitrogen, hydrogen, and ammonia. Composition stoichiometry describes the quantitative (mass) relationships among elements in compounds. For example, composition stoichiometry describes the nitrogen to hydrogen ratio in the compound ammonia: 1 mole of ammonia consists of 1 mole of nitrogen and 3 mole of hydrogen. As the nitrogen atom is about 14 times heavier than the hydrogen atom, the mass ratio is 14:3, thus 17 kg of ammonia contains 14 kg of nitrogen and 3 kg of hydrogen.A stoichiometric amount or stoichiometric ratio of a reagent is the optimum amount or ratio where, assuming that the reaction proceeds to completion: 1. All of the reagent is consumed, 2. There is no shortfall of the reagent, 3. There is no excess of the reagent. A non-stoichiometric mixture, where reactions have gone to completion, will have only the limiting reagent consumed completely. Mole Relationships in a Chemical Reaction Mole relationships in a chemical reaction can be determined by looking at the balanced reaction equation as shown below for the reaction of aluminum (Al) with hydrochloric acid (HCl) to produce aluminum chloride (AlCl3) and hydrogen gas (H2): 2Al + 6HCl = 2AlCl3 + 3H2 A balanced reaction equation has numbers in front of each substance called coefficients. If there is no number in front of a substance, assume the coefficient to be 1. These coefficients tell us the ratio of how many elements or molecules of each substance will be consumed and produced in that chemical reaction. From the reaction equation above, we can see that for every 2 moles of Al, we will produce 2 moles of AlCl3. This mole relationship can also be used as a conversion factor. There are several conversion factors that we can derive from this balanced reaction equation:

Usin The to mthe r

Or,

ng the Mole

mole to molmoles of anotreaction abov

if you want

Relationshi

le relationshther substancve, how man

to produce 4

ip as a Conv

hips or equalce in the samny moles of H

4.0 moles of

version Fac

ities can be me chemical H2 gas woul

f AlCl3, how

40

ctor

used as a coreaction. Fo

ld be produc

much Al wo

onversion facor example, ed?

ould you nee

ctor betweenif you starte

ed to start w

n moles of oed with 1.0 m

with?

one substancemole of Al in

e n

41


INVESTIGATING STOICHIOMETRY WITH SODIUM SALTS OF CARBONIC ACID Prelaboratory Assignment

Prepare a typed pre-lab report on the Investigating stoichiometry with Sodium Salts of Carbonic Acid

experiment to be turned in at the beginning of the lab period. This experiment should include:

(1) A summary of the concept(s).

(2) Chemical reactions of NaHCO3 and Na2CO3 with HCl.

(3) Calculations of NaCl theoretical yield values when:

A. 0.15 g of NaHCO3 reacts with HCl.

B. 0.15 g of Na2CO3 reacts with HCl.

Postlaboratory (Lab Report) Assignment Write a lab report on the Investigating stoichiometry with Sodium Salts of Carbonic Acid. The lab report will

consist of the Introduction, Experimental Procedure, Results, Discussion and Conclusion sections. Follow

the lab report guidelines from the syllabus (Appendix I) and the grading rubric of this experiment (next page,

this document).

42

Investigating stoichiometry with Sodium Salts of Carbonic Acid Lab Report Experiment 7

Grading Rubric (100 points) PRE-LAB REPORT (25 points) (1) A summary of the concept(s). (10 points)

(2) Chemical reactions of NaHCO3 and Na2CO3 with HCl. (5 points)

(3) Calculations of NaCl theoretical yield values when: (10 points)

A. 0.15 g of NaHCO3 reacts with HCl.

B. 0.15 g of Na2CO3 reacts with HCl.

LAB REPORT (75 points) Introduction (10 points) Includes problem, background information, hypothesis and expected results. Experimental Procedure (10 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional sources. Results (20 points) (1) Presents table(s) and calculations clearly and accurately. All data (grams of unknown samples, mL HCl, theoretical and actual yield of NaCl, etc..) are included. No hand drawn figures/tables are accepted. (2) Contents statements for each table/calculation and summarizes the overall trend shown by your data. Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ... Conclusions (15 points) State what it was learnt about the scientific concept(s) of the lab from doing the experimental procedure; give enough details of what it was accomplished to be convincing. Include in your conclusions a section that summarizes all of your data and conclusions.

43


DETERMINATION OF % COMPOSITION OF PENNIES USING REDOX AND DOUBLE DISPLACEMENT (PRECIPITATION) REACTIONS

The main purpose of the lab is to determine the percent composition of pennies using oxidation reduction and double displacement reactions and titration techniques. The quantitative determination of zinc is conducted by precipitation followed by isolation and weighing of the Zn(OH)2 precipitate (gravimetric analysis). In the first part of this experiment you dissolve the zinc core of a penny and leave the copper covering intact by putting four notches in the coin using a triangular file and placing the penny in 50 ml of a predetermined* concentration of HCl solution overnight (this would have been done for you by a different group on a previous day but you need to do this for a different group for the following day). *Determine the concentration needed to add ~ 2 times the numbers of moles of HCl needed in 50 ml of solution to completely react with all of the Zinc based on “known” percentages (grams) of Zn in pennies. You will then determine the percent copper from the mass of the copper and the percent zinc by two methods (precipitation and titration).

x Part Ia: Titration experiment o If a known amount of HCl is added to a beaker that the penny was placed into, then one could

determine the amount of HCl that was used up in the reaction of HCl with Zinc. Based on that information design an experiment to calculate the % zinc in a penny.

x Part Ib: Precipitation experiment o Note that when zinc reacts with NaOH in the titration experiment described above, zinc

hydroxide is formed. According to the solubility rules (see pages 4-5), zinc hydroxide should be insoluble. Based on that information design an experiment to calculate the % zinc in a penny.

Answer the questions on the pre-lab guide (next two pages) to help guide the titration and precipitation experiments.

44

Determination of % Composition of Pennies Using Redox and Double Displacement (Precipitation) Reactions

Questions to answer to help guide the development of procedures for the determination of the percent copper and zinc in pennies through titration and gravimetric techniques: 1. What is the weight of a post 1982 penny? 2. What is the percent copper and zinc in a post 1982 penny? 3. How many grams of copper and zinc are in a post 1982 penny? 4. How many moles of copper and zinc are in post 1982 pennies? 5. Write a balanced reaction of zinc with HCl. 6. How many moles of HCl are needed to react completely with all of the zinc in a post 1982 penny? 7. In a procedure developed to determine the percent zinc in post 1982 pennies, 50 ml of an HCl solution was used to react (dissolve) all of the zinc in the penny. To ensure complete reaction, the solution contains twice as many moles of HCl that is actually needed. What concentration of HCl should be used? 8. In the scenario described in problem 7, what is the amount (in moles) of excess (unreacted) HCl in solution? 9. How many moles of NaOH would be needed to completely react with all of the excess HCl determined in problem 8?

45

10. As described in problem 7, a procedure was developed to determine the percent zinc in post 1982 pennies. In that procedure 50 ml of an HCl was used to react (dissolve) all of the zinc in the penny. To ensure complete reaction, the solution contains twice as many moles of HCl that is actually needed. To determine the percent zinc in the penny, the excess (unreacted) HCl was titrated with NaOH. Determine the concentration of NaOH needed if you want to use approximately 25 mL of NaOH to titrate the excess HCl. 11. Write the balanced chemical reaction of zinc with HCl (same as problem 5). Is the product of this reaction soluble in aqueous solution? 12. Write the balanced chemical reaction of the product of the reaction described above (problem 11) with NaOH. Is the product of this reaction soluble in aqueous solution?

46


DETERMINATION OF % COMPOSITION OF PENNIES USING REDOX AND DOUBLE DISPLACEMENT (PRECIPITATION) REACTIONS


Prepare a typed pre-lab report on the Determination of % Composition of Pennies Using Oxidation-

Reduction and Precipitation Reactions experiment to be turned in at the beginning of the lab period. This

experiment should include:

Objective(s)

Questions 1- 12 Pre-lab guide (typed, show calculations)

Step by step procedure of the experiment


Write a lab report on the Determination of % Composition of Pennies Using Oxidation-Reduction and

Precipitation Reactions. The lab report will consist of the Introduction, Experimental Procedure, Results,

Discussion and Conclusion sections. Follow the lab report guidelines from the syllabus (Appendix I) and the

grading rubric of this experiment (next page, this document).

47

Determination of % Composition of Pennies Using Oxidation-Reduction and Precipitation Reactions Lab Report Experiment 8

Grading Rubric (100 points) PRE-LAB REPORT (25 points) Objective(s) (5 points) Questions 1- 12 Pre-lab guide (typed, show calculations) (10 points) Step by step procedure of the experiment (10 points) LAB REPORT (75 points) Introduction (10 points) Includes problem, background information, hypothesis and expected results. Experimental Procedure (15 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional sources. Results (20 points) (1) Presents table(s) and calculations clearly and accurately. All data: theoretical and actual yield of Cu, Zn and Zn(OH)2, percent copper and zinc are included. No hand drawn figures/tables are accepted. (2) Shows acceptable errors of precision measurements. (3) Contents statements for each table/calculation and summarizes the overall trend shown by your data. Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the validity of the data collected (compare them with the literature values). Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ... Conclusion (10 points) State what it was learnt about the scientific concept(s) of the lab from doing the experimental procedure; give enough details of what it was accomplished to be convincing. Include in your conclusions a section that summarizes all of your data and conclusions.

48


DETERMINATION OF % COMPOSITION OF PENNIES USING SPECTROSCOPY

INTRODUCTION

Complex ions are ions formed by the bonding of a metal atom or ion to two or more ligands by coordinate covalent bonds. A ligand is a negative ion or neutral molecule attached to the central metal ion in a complex ion. Many of these species are highly colored due to their ability to absorb light in the visible region of the electromagnetic spectrum. In this experiment, you will first dissolve a penny in a concentrated solution of nitric acid, HNO3. In aqueous solution, most of the first-row transition metals form octahedral complex ions with water as their ligands as shown for copper:

Cu(s) + 4 HNO3(aq) + 4 H2O(l) → Cu(H2O)62+(aq) + 2 NO2(g) + 2 NO3

-(aq)

IMPORTANT: When copper is oxidized by concentrated nitric acid, HNO3, to produce Cu2+ ions; the nitric acid is reduced to nitrogen dioxide (NO2), a poisonous brown gas with an irritating odor (SO DO THIS IN THE HOOD):

Once the penny has been dissolved, you will then convert the aquated copper complex ions to their tetraamine complex ions (ie., by replacing the H2O ligands with ammonia, NH3, ligands) as shown below:

Cu(H2O)62+(aq) + 4 NH3(aq) → Cu(NH3)4

2+(aq) + 6 H2O(l)

You can detect the presence of the Cu(NH3)42+ ion by its characteristic deep-blue color, and you can

measure its intensity with a spectrophotometer as a way to determine the percentage of copper in a penny. (Zinc has similar reactions to those of copper above, but does not form a colored complex ion solution and is not able to be analyzed with a visible spectrophotometer.)

To do this you will first need to construct a calibration curve that relates the measured absorbance, A, to known concentrations of the Cu(NH3)4

2+ ion using the Beer-Lambert Law. Concentration and absorbance are related according to the Beer-Lambert Law

A = a�b�c

where ‘A’ is the absorbance of the species, ‘a’ is the molar absorptivity (a constant that indicates how well the species absorbs light of a particular wavelength, in units of M-1 cm-1), ‘b is the path length that the light must travel through the solution (1.00 cm for the cuvette), and c is the concentration (in mol/L).

Then, you can use the calibration curve to determine the concentration of Cu(NH3)42+ in the solution

prepared from your penny, and from that concentration determine the percent copper in the penny through a

49

series of calculations. In the density experiment you used the linear relationship of % Cu or % Zn to the density of the Cu/Zn mixtures to develop a calibration curve. In this experiment you will use the linear relationship of the absorbance (y-axis) of light with concentration (x-axis) to develop a calibration curve.

PROCEDURE

PART I: PREPARING SAMPLE (PENNY) FOR ANALYSIS

1. Obtain a penny minted after 1982, and RECORD the mass. Place your penny in a 150 mL beaker and label the beaker, and obtain a watch glass to rest on top of the beaker.

2. In the fume hood add 20 mL of 8 M HNO3 in a graduated cylinder to the beaker containing the penny. The reaction of the copper and zinc metals in the penny with HNO3 is quite vigorous so you will not need to stir the reaction. RECORD your observations of the reaction, cover the beaker with a watch glass, and allow the reaction to go to completion in the fume hood.

3. After reaction is complete, carefully transfer this solution to a 100 mL volumetric flask. Use small portions of distilled water to rinse down the watch glass and the sides of the beaker and transfer the washes to the same 100 mL volumetric flask. Add distilled water to the mark, cover the flask with a small square of Parafilm. Invert several times to mix with your gloved thumb covering the top of the flask so that the liquid does not pour out.

4. Obtain three clean 25 mL beakers, or some other appropriate container, and label them 6, 7, and 8. Transfer 4.00 mL of the penny solution to each of the beakers.

5. In the fume hood, you will add 1.0 mL of 15 M NH4OH carefully to each beaker until the light-blue precipitate that initially forms dissolves and a deep-blue solution results. Then, add 5.0 ml of distilled water and mix thoroughly.

6. Discard the remaining solutions in the labeled waste container in the fume hood. Rinse the flasks with small portions of distilled water and discard the rinses.

PART II CONSTRUCTION OF A CALIBRATION CURVE FOR Cu(NH3)42+

7. Record the concentration of the already prepared stock solution in your notebook. 8. Obtain five clean 25 mL beakers (or some other suitable container) and number them 1-5. Using

a clean graduated pipet, or some other quantitative method, add the amount of stock solution as indicated in the table to the right.

9. In the fume hood note the reactions that take place as you add 1.0 mL of 15 M NH4OH to each volumetric flask – a light-blue precipitate that initially forms should dissolve resulting in a deep-blue solution.

10. Add the appropriate amount of water to get each of the solutions to 10 ml, (see Table to the right). After adding the appropriate amount of water, mix the solutions thoroughly. These solutions are your standard solutions for the calibration curve.

11. Obtain two cuvettes. One will be used to calibrate the spectrophotometer the other for the standard solutions. This will be explained to you by the Instructor.

Beaker # mL stock Cu2+

solution

mL of conc.

NH4OH

mL of H2O

1 1 1.0 8 2 2 1.0 7 3 3 1.0 6 4 4 1.0 5 5 5 1.0 4

50

Determination of % Composition of Pennies Using Spectroscopy

Questions to answer to help guide the development of procedures for the determination of the percent copper and zinc in pennies via spectroscopy. Maria was given an assignment by one of her chemistry instructors to verify the % composition of copper in pennies using spectroscopy. To do so she used the following procedures:

1. She dissolved the penny in 20 mL of 8M nitric acid. 2. After the penny was completely dissolved she brought the volume of the solution to 100 mL. 3. Next she took 4 mL of that solution, and added 1 mL of 15 M NH3 then 5 mL of water. 4. After which, she prepared various concentrations of Cu(NH3)4

2+ solutions, as shown in the table below to be used as standards to determine the % composition of copper pennies. The concentration of the stock solution used to prepare the standards was 15.5 mM Cu2+.

Beaker # mL stock

Cu(NH3)42+

solution mL of conc. NH3 mL of H2O

1 1 1 8 2 2 1 7 3 3 1 6 4 4 1 5 5 5 1 4

Composition of Penny: 2.5 % copper 97.5% zinc Weight of coin: 2.5 grams

Answer the questions below related to the development of procedures Maria should use to determine the % copper in the coin.

1. What is the weight of a post 1982 penny?

2. What is the % copper in a post 1982 penny?

3. How many grams of copper are in a post 1982 penny?

4. How many moles of copper are in post 1982 pennies?

51

5. Write all pertinent balanced reactions of copper and zinc with HNO3.

6. Write all pertinent balanced reactions of copper and zinc with NH3.

7. What is the concentration of the copper in solution after dissolving the penny in HNO3 and bringing the volume up to 100 mL?

8. What is the concentration of the copper in solution after taking 4 mL of the solution from problem 7, adding 1.0 mL NH3 then adding 5.0 mL of water?

9. In order to determine the amount of copper in a penny various concentrations of copper solutions were made for use as standards in preparation of a calibration curve. The concentration of the stock solution used to prepare the standards was 0.0155 M Cu2+. The calibration standards were prepared as shown in the table below. Calculate the concentration of each of the standards in molarity using the MiVi = MfVf formula.

Beaker # mL stock Cu2+ solution mL of conc. NH3 mL of H2O Final Concentration

of Cu2+

1 1 1 8 2 2 1 7 3 3 1 6 4 4 1 5 5 5 1 4

52

When the spectroscopic data from the experiment was plotted (see graph below) the equation for the line was, as shown in the graph, (y = 51.613+0.0078). If the average absorbance for samples was 0.261 absorbance units, what is the concentration of the copper in the sample solution?

y = 51.613x + 0.0078

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009

Ab

sorb

ance

s

Concentration Cu2+

Determination of Cu2+ in Pennies

53


DETERMINATION OF % COMPOSITION OF PENNIES USING SPECTROSCOPY

Prelaboratory Assignment Prepare a typed pre-lab report on the Determination of % Composition of Pennies Using

Spectroscopy experiment to be turned in at the beginning of the lab period. This experiment

should include:

Summary of the concept(s)

Questions 1- 9 Pre-lab guide (typed, show calculations)


Write a lab report on the Determination of % Composition of Pennies Using Spectroscopy. The

lab report will consist of the Introduction, Experimental Procedure, Results, Discussion and

Conclusion sections. Follow the lab report guidelines from the syllabus (Appendix I) and the

grading rubric of this experiment (next page, this document).

54

Determination of % Composition of Pennies Using Spectroscopy Lab Report Experiment 9

Grading Rubric (100 points) PRE-LAB REPORT (25 points) Summary of the concept(s) (10 points) Questions 1- 9 Pre-lab guide (typed, show calculations) (15 points) LAB REPORT (75 points) Introduction (10 points) Includes problem, background information, hypothesis and expected results. Experimental Procedure in the Lab report (15 points) Provides a concise, easy-to-follow description of the procedures followed in the lab. Includes enough details so that the experiment can be repeated without the aid of additional sources. Data & Results in the Lab Report (20 points) (1) Presents table(s) and calculations clearly and accurately. All data: calibration curve (table and graph), theoretical and actual yield of Cu, precision of copper concentration determinations) are included. No hand drawn figures/tables are accepted. (2) Shows acceptable errors of precision measurements. (3) Contents statements for each table//calculation and summarizes the overall trend shown by your data. Discussion (20 points) Subjective evaluation of your own execution of the experiment, the performance of the instruments and materials used, the reproducibility and validity of the data collected. Results are clearly stated. Data: Are the data reasonable? Do they coincide with theory? For unreasonable data ask WHY? Where there any mistakes? Was the experiment performed improperly? ... Conclusion (10 points) State what it was learnt about the scientific concept(s) of the lab from doing the experimental procedure; give enough details of what it was accomplished to be convincing. Include in your conclusions a section that summarizes all of your data and conclusions.

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Comptamucc / Lab Experiments Fall 2015 Chem One