Chapter 9Chapter 9Calculations from Chemical Calculations from Chemical

EquationsEquations(Stoichiometry)(Stoichiometry)

Objectives:•The Mole-Ratio Method•Mole-Mole Calculations•Mole-Mass Calculations•Mass-Mass Calculations•Limiting-Reactant and Percent Yield Calculations

A Short Review• Molar Mass

– Sum of the atomic masses of all atoms in a molecule

• Molecules and Moles– A molecule is the smallest unit of a substance– A mole is Avogadro’s number (6.022 x 1023)

molecules of that substance

• Balanced Equations– Equations must be balanced– Number in front of a formula in a balanced

chemical equation represent the number of moles of that substance

The Mole-Ratio Method

• Stoichiometry– quantitative relationships among reactants and

products

• Mole ratio– Ratio between the number of moles of any two

species involved in a chemical reaction

– 2H2 + O2 2H2O

– Six mole ratios can be written:

2 mol H2 2 mol H2O

2 mol H2 1 mol O2

1 mol O2 2 mol H2

1 mol O2 2 mol H2O

2 mol H2O 2 mol H2

2 mol H2O 1 mol O2

The Mole-Ratio Method

• Use the mole ratio to convert number of moles of one substance to number of moles of another substance

• How many moles of H2O can be obtained from 4.0 moles of O2?

4.0 mol O2 2 mol H2O

1 mol O2

= 8.0 mol H2O

The Mole-Ratio Method

• Three basic steps:– Convert the quantity of starting

substance to moles (if it is not given in moles)

– Convert the moles of starting substance to moles of desired substance

– Convert the moles of desired substance to the units specified in the problem

Grams of A

Atoms or Moleculeshof A

Moles of A Moles of B

Grams of B

Atoms or Moleculesof A

Quantity of Given(mass, atoms, molecules)

Quantity of Unknown(mass, atoms, molecules)

MolesOf

Given

MolesOf

Unknown

Convert to Moles

Find molar ratio

Convert to desired

units

Mole-Mole Calculations

• Quantity of given substance in moles• Quantity of desired substance requested in

moles• How many moles of carbon dioxide will be

produced by the complete reaction of 2.0 mol of glucose (C6H12O6) according to the following reaction?

• C6H12O6 + 6O2 6CO2 + 6H2O

2.0 mol C6H12O6 6 mol CO2

1 mol C6H12O6

= 12 mol CO2

Mole-Mole Calculations

N2H4 + 2H2O2 N2 + 4H2O• If you have 3 moles of N2H4 how many moles of N2

will you produce?3 mol N2H4 1 mol N2 =

1 mol N2H4

• How many molecules of H2O2 are needed to produce 3 moles of H2O?

3 mol H2O 2 mol H2O2 6.022 X 1023 molecules

4 mol H2O 1 mol H2O2

3 mol N3 mol N22

= 9 x 10= 9 x 102323 molecules Hmolecules H22OO22

Mole-Mass CalculationsMole-Mass Calculations

• Calculate the mass of one substance when given moles of another

>> OR <<

• Calculate the moles of one substance when given mass of another

• Mass of substance 1 to moles of substance 1 to moles of substance 2 (using mole ratio)

Mole-Mass CalculationsMole-Mass Calculations

2KClO3 2KCl + 3O2

• How many grams of potassium chlorate are needed to produce 5 moles of oxygen?

• How many moles of potassium chloride can be produced from 100.0 g of potassium chlorate?

Mass-Mass CalculationsMass-Mass Calculations

• Calculate the mass of one substance when given the mass of another

• Mass of substance 1 moles of substance 1 moles of substance 2 mass of substance 2

Mass-Mass CalculationsMass-Mass Calculations

2AgNO3 + H2S Ag2S + 2HNO3

• How many grams of silver nitrate are required to produce 250.0 grams of silver sulfide?

• How many grams of nitric acid will be produced if 325 grams of H2S is used?

Mass-Mass CalculationsMass-Mass Calculations

• What mass of water is produced by the complete combustion of 225.0 g of butane (C4H10)?

– Write reaction– Balance– Solve problem

• 2C4H10 + 13O2 8CO2 + 10H2O

Part One Homework

• Paired Exercises # 3-19 odd

• Additional Exercises #33 & 37

• Enjoy your Thanksgiving!!!

• See you Tuesday, December 2nd

• Be prepared for a QUIZ over this information!

Limiting Reactants• Often, quantities of reactants are not

perfect– One may be left over– Amount of product formed depends on reactant

that is not in excess• Limiting reactant

• One batch of chocolate chip cookies requires 4 eggs, 4 cups of flour, and 12 oz. of chocolate chips and produces 45 cookies.– How many batches can be made from 1 dozen

eggs, 15 cups of flour, and 36 oz. of chocolate chips?

– How many cookies will be produced from 8 eggs, 4 cups of flour, and 24 oz. of chocolate chips?

3 batches (eggs are limiting reactant)

Flour is limiting reactant, only 1 batch, so 45 cookies

Limiting Reactants

H2 + Cl2 2HCl

• How many grams of hydrogen chloride can be produced from 0.490 g of hydrogen and 50.0 g of chlorine?– Calculate mass-mass for BOTH

reactants– The one with the LEAST product is your

limiting reactant (and thus your answer)

Limiting Reactants

• If 36.5 g HCl is reacted with 85.6 g of Ba(OH)2 which is the limiting reactant? How much BaCl2 will be produced?– Barium hydroxide is the limiting reactant

– 104 g BaCl2 will be produced

Percent Yield

• Thus far, quantities represent maximum yield (100%)

• Actual yield in lab is not often 100%– Side reactions– Many reactions reversible– Poor lab skills

• Expected (theoretical) yield– calculated amount of product that can be

obtained

• Actual yield– Amount actually obtained

Percent Yield

• Percent yield– Ratio of actual yield to theoretical yield

Actual yield x 100 = percent yield

Expected yield

• Determine the percent yield for the reaction between 2.80 g Al(NO3)3 and excess NaOH if 0.966 g Al(OH)3 is recovered. (your second product is NaNO3

Percent Yield

• A student places an iron nail with a mass of 2.32 g into a flask of CuSO4. The nail reacts completely, leaving a quantity of copper metal in the bottom of the flask. The student finds the mass of the recovered copper to be 2.51 g. The equation for this reaction is: Fe + CuSO4 FeSO4 + Cu– What is the expected yield?– What is the percent yield?

Percent Yield

• When octane (C8H18) is burned in oxygen, carbon dioxide and water are produced. If 320 g of octane are burned and 392 g of water are recovered, what is the percent yield of the experiment? – First, write and balance the equation– Then, answer the question

Percent Yield

• 4Al + 3O2 2Al2O3

• If 0.25 mol Al and 0.40 mol O2 are reacted, which is the limiting reactant?

• If a student reports a percent yield of 75.5%, how much product (in grams) did she recover?

Part Two Homework

• Questions # 1 & 2

• Paired Exercises # 21, 23, 27 & 29

• Additional Exercises # 31 & 41

• Be prepared for TEST on Thursday

• Last EXAM of the quarter…

• Remember, EC due December 9th