Chapter 9
Calculations from Chemical
Equations
(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
1 mol O2
2 mol H2
2 mol H2O
1 mol O2
2 mol H2O
2 mol H2O
2 mol H2
1 mol O2
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 B
Grams of A
Moles of A
Atoms or Molecules
hof A
Moles of B
Atoms or Molecules
of A
Quantity of Given
Quantity of Unknown
(mass, atoms, molecules)
(mass, atoms, molecules)
Convert
to Moles
Moles
Of
Given
Convert
to desired
units
Find molar ratio
Moles
Of
Unknown
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
=
3 mol N2
• 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
= 9 x 1023
molecules H2O2
Mole-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 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 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 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 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
3 batches (eggs are limiting reactant)
chips?
– How many cookies will be produced from 8
eggs, 4 cups of flour, and 24 oz. of chocolate
chips? 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