Stoichiometry
The Mathematics of
Chemical Equations
9.1: Balanced Chemical Equations
1. Provides qualitative and quantitative information.
2. Supports the Law of Conservation of Mass
3.
2H2 +
O2 → 2H2O
The above equation is interpreted in terms of particles
as follows:
A. 2 molecules of H2 react with 1 molecule of O2 to
produce 2 molecules of water.
The ratio of H2 to O2 to H2O is 2:1:2. OR
2H2
+ O2 → 2H2O
B. 20 molecules of H2 react with 10 molecules of O2
to produce 20 molecules of water.
Again, the ratio of H2 to O2 to H2O is 2:1:2.
2H2
+ O2 → 2H2O
C. The original equation can then be interpreted as
follows:
2 moles of H2 react with 1 mole of O2 to produce 2
moles of water.
The ratio of H2 to O2 to H2O is 2:1:2.
D. It is more convenient to interpret the coefficients
as the number of moles, because we measure
amounts of substances by grams in the laboratory.
We can convert between grams and moles in
solving problems involving chemical reactions.
4. Stoichiometry: The study of the mathematical
(quantitative) relationships that exist in a formula
and in a chemical reaction.
5. The word Stoichiometry comes from the Greek word
“stoicheion” meaning element, and “metron”,
meaning measure.
6. Importance: The safe, economical and
reproducible manufacture of chemicals (or food)
and the safe administration of pharmaceuticals.
7. Proof: The Law of conservation of mass is shown
by the balanced equation, or by adding up all the
masses of reactants and products to determine if
they are equal.
8. Mole-Mole Problems require 3 steps to determine
the molar ratio: Stoichiometryville Map
A. Write Equation
B. Balance the equation to determine the molar ratio
C. Moles Given (Known) to Moles Unknown
Mass
Mass
Given
Substance
Molar Mass
Molar Mass
Particles
Substance to be
determined
Particles
6.02 x 1023
6.02 x 1023
22.4 L
Volume
22.4 L
Volume
Ex #1) Ammonium nitrate decomposes into
dinitrogen monoxide and water. How many moles
of products are produced from 2.25 moles of
reactants?
Equation: NH4NO3  N2O + 2 H2O
Ratio:
1
: 1
:
2
2.25 mol NH4NO3
 1 mol NO2 
2.25 mol NH 4 NO3 
  2.25 mol N 2O
 1 mol NH 4 NO3 
Ex #2) Hydrochloric acid reacts with zinc in a single
replacement reaction, how many moles of HCl are
needed to react with 2.3 moles of Zn?
? mol
2.3 mol
Equation: 2 HCl + Zn  ZnCl2 + H2
Ratio:
2
: 1 :
1
: 1
 2 mol HCl 
2.3 mol Zn 
 = 4.6 mol HCl
 1 mol Zn 
Stoichiometry Problems:
1. A balanced equation represents a chemical reaction that
conforms to the Law of Conservation of Mass
2. Subscripts can NEVER be changed to balance a chemical
equation.
3. The total atoms/mass of the reactants is the same as the
total atoms/mass of the products.
4. Coefficients in a balanced equation relate moles of
substances in the reaction.
5. Coefficients are used in constructing the molar ratio for a
stoichiometric problem.
6. Grams of the known must first be converted to moles,
then use the molar ratio to relate the 2 substances, finally
convert the moles of the unknown to grams.
7. Mass-Mass Problems:
Ex#1)What mass of aluminum oxide can be prepared by the reaction of
67.5 g of Al and oxygen?
67.5 g
4Al
?g
+
3O2
→
2Al2O3
 1 mol Al   2 mol Al2O3   102.0 g Al2O3 
67.5 g Al 
 = 128 g Al2O3


 27.0 g Al   4 mol Al   1 mol Al 2O3 
Ex#2)Glucose is used as an energy source by the human body. Calculate
the number of grams of oxygen needed to oxidize (burn) 12.5 g of
glucose. Write the balanced combustion reaction.
12.5 g
?g
C6H12O6 + 6O2 → 6CO2 + 6H2O
 1 mol C6 H12O6  6 mol O2  32.0 g O2 
12.3 g C6 H12O6 


 = 13.3 g O2
 180.0 g C6 H12O6  1 mol C6 H12O6   1 mol O2 
Ex #3) During WWII, Germany used a method called the Haber Process
to produce ammonia for explosives. Today a similar process is used to
make fertilizers. If excess of nitrogen is reacted with 1.71 mol of
hydrogen, how many grams of ammonia, NH3 is produced?
N2
+ 3 H2 → 2 NH3
1.71 mol
?g
 2 mol NH3   17.0 g 
1.71 mol H 2 

 = 19.4 g NH3
 3 mol H 2   1 mol NH3 



Ex #4) iron will react with oxygen gas to produce iron (III) oxide. How
many grams of iron (III) oxide will be produced if 0.18 mol of iron reacts?
4Fe + 3O2 → 2Fe2O3
.18 mol
 2 mol Fe2O3
0.18 mol Fe 
 4 mol Fe

?g
  159.6 g Fe O 
2 3

 = 14 g Fe2O3
  1 mol Fe2O3 


8. Mass-Volume Problems: Airbags contain a powder called sodium azide
(NaN3). When exposed to a spark the sodium azide quickly decomposes into
sodium and nitrogen gas. If an uninflated airbag contains 125 g of NaN3, what
volume in liters of nitrogen gas will be produced at STP?
2 NaN3
+
2 Na
→
125 g
 1 mol NaN3
125 g NaN3 
 65.0 g NaN3

3 N2
?L
  3 mol N 2

  2 mol NaN3

  22.4 L N 
2

 = 64.6 L N 2
  1 mol N 2 


9. Volume-Volume Problems: What volume in liters of hydrogen gas is
needed to react completely with 15.5 L of nitrogen gas to produce
ammonia gas (NH3) at STP?
3 H2 + N2 →
2 NH3
?L
15.5 L
 3 mol H 2 
15.5 L N 2 
 = 46.5 L H 2
 1 mol N 2 


5. The limiting reactant is the starting substance that
becomes used up first when a chemical reaction occurs. It
controls how much or how little product can be formed.
Ex) If a recipe for 1 cake is:
and your kitchen contains:
2 cups flour
2 eggs
1 cup sugar
1½Tbls. baking powder
1 cup water
⅓ cup oil
7 cups flour
2 dozen eggs
9 cups sugar
10 Tbls. baking pwdr
unlimited water
3⅓ cups oil
• How many cakes can you make given the ingredients
present in your kitchen? 3 What ingredient is the
limiting reactant? flour How many eggs are left over
after making the cakes? 18
6.
4 NH3 + 5 O2 → 4 NO + 6 H2O
3.25g
3.50g
?g
 1 mol NH 3  4 mol NO   30 g NO 
3.25 g NH 3 


 = 5.74 g NO
 17 g of NH 3  4 mol NH 3   1 mol NO 
 1 mol O2  4 mol NO   30 g NO 
3.50 g O2 


 = 2.63 g NO
 32 g of O2  5 mol O2   1 mol NO 
What is the most NO you can make? ______
2.63 g
oxygen
Which reactant is the limiting reactant? ________
Reactants → Products