Stoichiometry: Stoichiometry: Quantitative Quantitative Information About Information About Chemical ReactionsChemical Reactions

Chapter 4Chapter 4

4.1 Mass Relationships4.1 Mass Relationships

stoichiometric factorstoichiometric factor: a mole ratio : a mole ratio based on the coefficients of reactants based on the coefficients of reactants or products in the balanced equationor products in the balanced equation

What mass of oxygen, OWhat mass of oxygen, O22, is required , is required to completely combust 454 g of to completely combust 454 g of propane, Cpropane, C33HH88? What masses of CO? What masses of CO22 and Hand H22O are produced?O are produced?

CC33HH88 + 5O + 5O22 3CO 3CO22 + 4H + 4H22OO

4.2 Limiting Reactants4.2 Limiting Reactants

A limiting reactant is determines the A limiting reactant is determines the amount of product formed. amount of product formed.

Step 1: Find the amount of each Step 1: Find the amount of each reactant.reactant.

Step 2: What is the limiting reactant?Step 2: What is the limiting reactant? Step 3: Calculate the mass of Step 3: Calculate the mass of

product.product. Step 4: Calculate the mass of excess Step 4: Calculate the mass of excess

reactant.reactant.

Limited ReactantsLimited Reactants

Pure silicon, required for computer Pure silicon, required for computer chips and solar cells, is made by chips and solar cells, is made by the reactionthe reaction

SiClSiCl44 + 2Mg + 2Mg Si + 2MgCl Si + 2MgCl22

If you begin with 225 g each of SiClIf you begin with 225 g each of SiCl44 and Mg, which is the limiting and Mg, which is the limiting reactant in this reaction? What reactant in this reaction? What quantity of Si, in grams, can be quantity of Si, in grams, can be produced?produced?

4.3 Percent Yield4.3 Percent Yield

% yield = % yield = actual yieldactual yield x 100x 100 theoretical yieldtheoretical yield

Methanol, CHMethanol, CH33OH, can be burned in oxygen to OH, can be burned in oxygen to provide energy, or it can be decomposed to provide energy, or it can be decomposed to form hydrogen gas, which can then be used form hydrogen gas, which can then be used as a fuel.as a fuel.

CHCH33OH OH 2H 2H22 + CO + COIf 125 g of methanol is decomposed, what is If 125 g of methanol is decomposed, what is

the theoretical yield of hydrogen? If only the theoretical yield of hydrogen? If only 13.6 g of hydrogen is obtained, what is the 13.6 g of hydrogen is obtained, what is the percent yield of the gas?percent yield of the gas?

4.4 Quantitative 4.4 Quantitative AnalysisAnalysis A substance of unknown A substance of unknown

concentration may be allowed to concentration may be allowed to react with a known quantity of react with a known quantity of something elsesomething else

OROR An unknown substance can be An unknown substance can be

converted to one or more substances converted to one or more substances of known composition (back track)of known composition (back track)

Chemical AnalysisChemical Analysis

One method for determining the purity of One method for determining the purity of a sample of titanium (IV) oxide, TiOa sample of titanium (IV) oxide, TiO22, an , an important industrial chemical, is to important industrial chemical, is to combine the sample with bromine combine the sample with bromine trifluoride.trifluoride.3TiO3TiO22 + 4BrF + 4BrF33 3TiF 3TiF44 + 2Br + 2Br22 + 3O + 3O22

Oxygen gas is evolved quantitatively. The Oxygen gas is evolved quantitatively. The gas can be captured readily, and its mass gas can be captured readily, and its mass can be determined. Suppose 2.367 g of a can be determined. Suppose 2.367 g of a TiOTiO22-containing sample evolves 0.143 g of -containing sample evolves 0.143 g of OO22. What is the mass percent of TiO. What is the mass percent of TiO22 in in the sample?the sample?

Determining an Determining an Unknown FormulaUnknown Formula The unknown pure compound is The unknown pure compound is

decomposed into known products.decomposed into known products. The reaction products are isolated in The reaction products are isolated in

pure form and the amount of each is pure form and the amount of each is determined.determined.

The amount of each product is related The amount of each product is related to the amount of each element in the to the amount of each element in the original compound to give the original compound to give the empirical formula.empirical formula.

Determining an Determining an Empirical FormulaEmpirical Formula A 0.523 g sample of the unknown A 0.523 g sample of the unknown

compound Ccompound CxxHHyy is burned in air to is burned in air to give 1.612 g of COgive 1.612 g of CO22 and 0.7425 g and 0.7425 g of Hof H22O. A separate experiment O. A separate experiment gave a molar mass for Cgave a molar mass for CxxHHyy of 114 of 114 g/mol. Determine the empirical g/mol. Determine the empirical and molecular formulas for the and molecular formulas for the hydrocarbon.hydrocarbon.

4.5 Concentrations of 4.5 Concentrations of Compounds in SolutionCompounds in Solution 1 mole element = 6.02 x 101 mole element = 6.02 x 102323

atomsatoms 1 mole molecular compound = 1 mole molecular compound =

6.02 x 106.02 x 102323 molecules molecules 1 mole ionic compound = 6.02 x 1 mole ionic compound = 6.02 x

10102323 formula units formula units

MolarityMolarity

Molarity = moles of solute per liter Molarity = moles of solute per liter of solutionof solution

Example: Prepare 2.00L of 0.250M Example: Prepare 2.00L of 0.250M NaOH from solid NaOH.NaOH from solid NaOH.– Place 20.0g NaOH in a 2L volumetric Place 20.0g NaOH in a 2L volumetric

flask; add water to dissolve the NaOH, flask; add water to dissolve the NaOH, and fill to the mark with water, mixing and fill to the mark with water, mixing several times along the way. several times along the way.

MolarityMolarity

Sodium bicarbonate, NaHCOSodium bicarbonate, NaHCO33, is , is used in baking powder formulations used in baking powder formulations and in the manufacture of plastics and in the manufacture of plastics and ceramics, among other things. and ceramics, among other things. If 26.3 g of the compound is If 26.3 g of the compound is dissolved in enough water to make dissolved in enough water to make exactly 200mL of solution, what is exactly 200mL of solution, what is the molar concentration of NaHCOthe molar concentration of NaHCO33? ? What are the concentrations of the What are the concentrations of the ions?ions?

Preparations of Known Preparations of Known SolutionsSolutions An experiment in your laboratory An experiment in your laboratory

requires 250.mL of a 0.0200M requires 250.mL of a 0.0200M solution of AgNOsolution of AgNO33. You are given . You are given solid AgNOsolid AgNO33, distilled water, and a , distilled water, and a 250.mL volumetric flask. 250.mL volumetric flask. Describe how to make up the Describe how to make up the required solutionrequired solution

DilutionsDilutions

MM11VV11 = M = M22VV22

Prepare 2.00L of 0.250M NaOH Prepare 2.00L of 0.250M NaOH from 1.00M NaOH.from 1.00M NaOH.

Add 500.mL of 1.00M NaOH stock Add 500.mL of 1.00M NaOH stock solution to a 2L volumetric flask; solution to a 2L volumetric flask; add water, mixing until the flask add water, mixing until the flask is filled to the mark on the neck of is filled to the mark on the neck of the flask.the flask.

Preparing a Solution Preparing a Solution by Dilutionby Dilution In one of your laboratory In one of your laboratory

experiments, you are given a experiments, you are given a solution of CuSOsolution of CuSO44 that has a that has a concentration of 0.15M. If you mix concentration of 0.15M. If you mix 6.0mL of this solution with enough 6.0mL of this solution with enough water to have a total volume of water to have a total volume of 10.0mL, what is the concentration 10.0mL, what is the concentration of CuSOof CuSO44 in this new solution? in this new solution?

Preparing a Solution Preparing a Solution by Dilutionby Dilution An experiment calls for you to use An experiment calls for you to use

250. mL of 1.00M NaOH, but you 250. mL of 1.00M NaOH, but you are given a large bottle of 2.00M are given a large bottle of 2.00M NaOH. Describe how to make the NaOH. Describe how to make the 1.00M NaOH in the desired 1.00M NaOH in the desired volume. volume.

4.6 pH, a 4.6 pH, a Concentration Scale Concentration Scale for Acids and Basesfor Acids and Bases pH can be calculated directly pH can be calculated directly

from hydrogen ion concentrationfrom hydrogen ion concentration pH = -log[HpH = -log[H++] ] Can use the antilog of pH to Can use the antilog of pH to

figure out hydrogen ion figure out hydrogen ion concentrationconcentration

[H[H++] = 10] = 10-pH-pH

pHpH

What is the pH of a solution of What is the pH of a solution of HCl, where [HCl] = 2.6 x 10HCl, where [HCl] = 2.6 x 10-2-2 M? M?

What is the hydrogen ion What is the hydrogen ion concentration in saturated concentration in saturated calcium hydroxide with a pH of calcium hydroxide with a pH of 12.45?12.45?

4.7 Stoichiometry of 4.7 Stoichiometry of Reactions in SolutionReactions in Solution If you combine 75.0 mL of 0.350M If you combine 75.0 mL of 0.350M

HCl and an excess of NaHCl and an excess of Na22COCO33, , what mass of COwhat mass of CO22 (in grams) (in grams) should be produced?should be produced?

TitrationsTitrations

A titration can be used to A titration can be used to determine the concentration determine the concentration of an unknown solution. A of an unknown solution. A substance, present in substance, present in unknown quantity, can be unknown quantity, can be allowed to react with a known allowed to react with a known quantity of another substance. quantity of another substance. If the stoichiometric ratio for If the stoichiometric ratio for their reaction is known, the their reaction is known, the unknown quantity can be unknown quantity can be determined.determined.

TitrationsTitrations

Acid-base indicators are used to tell Acid-base indicators are used to tell when the reaction is complete. when the reaction is complete.

When the solution changes color, the When the solution changes color, the equivalence point (also called end equivalence point (also called end point) has been reached. The point) has been reached. The amount of OHamount of OH-- equals the amount of equals the amount of HH++..

Amount of base (mol) = conc. of base (M) x volume of Amount of base (mol) = conc. of base (M) x volume of base (L)base (L)

Acid-Base TitrationAcid-Base Titration

A 25.0 mL sample of vinegar requires A 25.0 mL sample of vinegar requires 28.33 mL of a 0.953 M solution of 28.33 mL of a 0.953 M solution of NaOH for titration to the equivalence NaOH for titration to the equivalence point. What mass (in grams) of acetic point. What mass (in grams) of acetic acid is in the vinegar sample, and what acid is in the vinegar sample, and what is the concentration of acetic acid in is the concentration of acetic acid in the vinegar?the vinegar?

CHCH33COCO22H + NaOH H + NaOH NaCH NaCH33COCO22 + H + H22OO

TitrationsTitrations

Often the concentration of the base Often the concentration of the base is unknown and must be determined is unknown and must be determined through standardization.through standardization.– primary standard: a sample of solid acid primary standard: a sample of solid acid

or base is titrated with a solution of the or base is titrated with a solution of the base or acid to be standardizedbase or acid to be standardized

– titrate with another solution that is titrate with another solution that is already standardizedalready standardized

StandardizationStandardization

Hydrochloric acid can be purchased Hydrochloric acid can be purchased from chemical supply houses with a from chemical supply houses with a concentration of 0.100M, and such a concentration of 0.100M, and such a solution can be used to standardize solution can be used to standardize the solution of a base. If titrating the solution of a base. If titrating 25.00 mL of a sodium hydroxide 25.00 mL of a sodium hydroxide solution to the equivalence point solution to the equivalence point requires 29.67mL of 0.100M mL, requires 29.67mL of 0.100M mL, what is the concentration of the what is the concentration of the base?base?

Redox TitrationsRedox Titrations

A sample containing vitamin C is A sample containing vitamin C is titrated with Brtitrated with Br22, an oxidizing , an oxidizing agent. agent.

CC66HH88OO66 + Br + Br22 2HBr + C 2HBr + C66HH66OO66

A 1.00g “chewable” vitamin C A 1.00g “chewable” vitamin C tablet requires 27.85mL of 0.102M tablet requires 27.85mL of 0.102M BrBr22 for titration to the equivalence for titration to the equivalence point. What is the mass of vitamin point. What is the mass of vitamin C in the tablet?C in the tablet?

4.8 Spectrophotometry4.8 Spectrophotometry

It is possible to measure the extent It is possible to measure the extent of light absorption through a of light absorption through a solution that is colored, and then solution that is colored, and then you can relate this to the you can relate this to the concentration of the dissolved concentration of the dissolved solute. This method of solute. This method of quantitatively measuring light quantitatively measuring light absorption is called absorption is called spectrophotometryspectrophotometry..

Transmittance and Transmittance and AbsorbanceAbsorbance TransmittanceTransmittance is the ratio of the is the ratio of the

amount of light transmitted by or amount of light transmitted by or passing through the sample relative to passing through the sample relative to the amount of light that initially fell on the amount of light that initially fell on the sample (the incident light). the sample (the incident light).

AbsorbanceAbsorbance is defined as the negative is defined as the negative log of the transmittance. As log of the transmittance. As absorbance increases, transmittance absorbance increases, transmittance decreases. Absorbance increases as decreases. Absorbance increases as concentration increases.concentration increases.

The Beer-Lambert lawThe Beer-Lambert law

Absorbance increases as path Absorbance increases as path length increases.length increases.

The amount of light absorbed by a The amount of light absorbed by a solution can be used to determine solution can be used to determine the concentration of the absorbing the concentration of the absorbing molecule in that solution.molecule in that solution.

A = A = εε x l x c x l x c where where εε is the molar absorptivity is the molar absorptivity

Homework for Chapter Homework for Chapter 44 After reading chapter 4, you After reading chapter 4, you

should be able to do the should be able to do the following…following…

P. 201 (77-86)P. 201 (77-86)