1.07 Accuracy and Precision

Virtual Laboratory

The Plan

• Review how to read instruments with accuracy and precision

• Complete 1 Trial of each Part of the Virtual Lab via screen share/slides with teacher

• Use the data to calculate density using the rules of accuracy/precision and significant figures

• Discuss the analysis questions• Complete the lab on your own

Before we Begin

• Please open your virtual lab in the course

Create/Download the Lab Form

• Copy and Paste the lab form into a document or download the pre-made form from the Chemistry Resource Center https://sites.google.com/site/chemistryflvs/

READY?

Why use Significant Figures?

• The benefit of this agreement is the consistency in the way all scientists read and record measurements.

• You will always know exactly how many digits your measurement should have based on the instrument used.

• You will be able to determine what the smallest graduations were on the instruments used by others when they share their data with you.

Please Open the Virtual Lab

* We will begin with Part I: Density of an unknown liquid

How do you measure properly?

• When measuring you will determine the number of sig figs by estimating one digit past the smallest measurement, or graduation, on the measuring tool.

• How many digits should our measurement have here?

What is this measurement?

Use the slides starting with the largest to make the balance even.

Make your measurement and record data

Add liquid to the cylinder and read the amount

What do you think this volume is? Record in your Notes

Take the mass of the full cylinder

• Let’s read this together. Record in your Notes

Repeat!

• You will need to complete two more trials of this Part on your own

• Let’s keep moving!

Go back to the Menu

Move on to Part II: Density of an irregular shaped object

Mass is EASY here

Fill cylinder and record volume

How many digits should we have? What is the volume?

Place Solid into Liquid ~ Read and Record

• This is called the “displacement method”

NEXT!

• You will need to complete two more trials of this Part on your own.

• Let’s move on to Part III

Part III: Density of a regular-shaped solid

Mass the Object. Read and Record

• What is the mass?

Use the ruler to measure length, width and height• Remember the number digits is the same for the

instrument regardless of the measurement

What about width?

• What is this measurement?

Complete length on your own!

• After you measure the length, click Data Table to review all of your information.

• How do your measurements compare?

• Did you use the correct number of digits?

Trial 1 Data

Remember to go back and complete 2 more trials for each Part

Calculations

• We now need to use our data to complete the density calculations

• Density is mass divided by volume orD= M/V

• If you have downloaded the pre-made lab form it is color coded and numbered to help you

Part I Density

• Part I: Density of Unknown Liquid – Calculate the mass (g) of the liquid for each trial (A).

(Subtract the mass (g) of the empty graduated cylinder [1] from the mass (g) of the graduated cylinder with liquid [3])

• mass of the liquid = [3] - [1] = (A) g• Trial 1:35.58 g- 25.02g = ? How many significant figures

should our answer have?

Remember: Rules for Addition/Subtraction

• The final answer cannot have more places after the decimal than any of the given measurements.

• The final answer cannot have a final digit, which represents the uncertain or estimated place, farther to the right than any of the final digits in the measurements used.

• SO.. Our answer should have the same number of decimal places as our measurements when adding/subtracting.

Calculate Density

– Calculate the density (g/mL) of the unknown liquid (B) for each trial. (Divide the mass (g) of the liquid calculated above (A) by the volume (mL) of the liquid [2])

• (A) / [2] = (B) g/mL• Trial 1: Using our mass we just calculated

divide by volume from the data table.10.56g/8.32 mL= ? g/mL• How many digits should our answer have?

Remember: Rules for Multiplication/Division

• Only given measurements affect the number of sig figs allowed in the final answer.

• If you are only given one measurement, the total number of sig figs in that measurement equal the total number of sig figs allowed in your final answer.

• If you are given more than one measurement to be used in a calculation, the final answer must be rounded to the same total number of sig figs as the measurement that has the least total sig figs.

• SO.. Our answer should have the same number of figures as the least significant figures in our data set when multiplying/dividing.

• How do we know how many significant figures our data has? Print your reference sheet! You will need it for the whole course

Rules to Determine Sig Figs

What’s our density?

10.56g/8.32 mL= ? g/mL• This is division so we follow the rules for

division and use the least number of figures.

• 8.32 has 3 figures• Our answer is 1.27g/mL when properly

rounded• Remember to include your units or your

measurement is just a number!

Rules for Rounding up or down

• In order to follow the significant figures rules for calculations, it is sometimes necessary to round your answer or add zeros to the end of the answer to give it the proper number of sig figs.

Density for Trial 2

• Part II: Density of Irregular-Shaped Solid – Calculate the volume (mL) of the irregular-

shaped solid (C) for each trial. (Subtract the volume (mL) of the water [5] from the total volume (mL) of the water and solid [6])

• [6] – [5] = (C) mL• Trial 1:

59.2 mL – 54.9 mL = ?Check your rules!!

What’s the answer?

• Remember to follow the rules for subtraction.

• volume= 4.3 mL– Calculate the density (g/mL) of the irregular-

shaped solid (D) for each trial. (Divide the mass of the solid [4] by the volume of the solid calculated above (C))

• [4] / (C) = (D) g/mL• Trial 1: 38.718 g/4.3 mL = ?• 9.0 g/mL

Part III

-Calculate the volume (cm3) of the regular shaped solid (E) for each trial. (Multiply the length [8] cm × width [9] cm × height[10] cm for each trial to get the volume in the unit cm3.)

• [8] x [9] x [10] = (E) cm3

• Trial 1: 5.24cm x 3.00cm x 2.50cm =?• 39.3 cm3

Density for Part III

– Calculate the density (g/ cm3)of the regular-shaped solid (F) for each trial. (Divide the mass (g) of the solid [7] by the volume (cm3) calculated above (E))

• [7] / (E) = (F) g/ cm3

• Trial 1: 25.33 g/39.3 cm3

• 0.64 g/cm3

Analysis Questions

• #1~ How would you determine the proper number of significant figures of a liquid using a graduated cylinder?

Hmm.. Well, what did we have to do to read our two different cylinders in this lab?

Analysis Questions

• #2~ Can just one measurement be considered precise? Can just one measurement be considered accurate? Explain your answers completely.

- What is accuracy? Precision?

Analysis Questions

• #3~ In parts II and III of the lab you used different sized objects in each trial. Compare the density values that you calculated for these items, how do the three trials compare?

What was our density for Part II?What about Part III?How did our density compare? What the object in

Part II more or less dense then Part III?

Congrats!!

• Remember to go back and complete two more trials and calculations for all three Parts of this lab before submitting to your teacher.

• Great job!

Questions?