Chapter 9
Stoichiometry
Definition of “Stoichiometry”:
the mathematics of chemical equations
Important Concepts:
1. You MUST have a balanced equation!
2. The coefficients of the equation describe the
amounts of each substance in moles.
Ex:
2 H2
+
O2

2 H2 O
2 mol H2 + 1 mol O2  2 mol H2O
3. A balanced equation is a recipe for the reaction. It
MUST be followed!
Mole Ratios
A mole ratio is a comparison between the amounts of two
substances in a chemical reaction.
Ex: 2 H2 + O2  2 H2O
The mole ratio of H2 to O2 is _________.
The mole ratio of H2 to H2O is ________.
Ex: C3H8 + 5 O2  3 CO2 + 4 H2O
The mole ratio of C3H8 to O2 is ___________.
The mole ratio of CO2 to H2O is __________.
The mole ratio of O2 to H2O is ____________.
**A mole ratio MUST be used to solve ANY type of reaction
stoichiometry problem.
Types of Stoichiometry Problems
1) Mole – Mole
2) Mole – Mass
3) Mass – Mole
4) Mass – Mass
Mole– Mole Calculations
mol A  mol B
Ex: 2 Mg + O2  2 MgO
How many moles of O2 are needed to react with 0.80 moles Mg
to make MgO?
Ex: How many moles of NH3 will be produced from the reaction of
1.35 moles H2 with N2?
Mole – Mass Calculations
mol A  mol B  g B
Mg + 2 HCl  H2 + MgCl2
How many grams of H2 are produced when 1.25 moles Mg
react?
How many grams of S are needed to react with 0.395 moles Al to
make aluminum sulfide?
Mass – Mole Calculations
g A  mol A  mol B
C3H8 + 5 O2  3 CO2 + 4 H2O
How many moles of O2 are needed to produce 25.6 grams
CO2?
Cu + 4 HNO3  Cu(NO3)2 + 2 H2O + 2 NO2
How many moles of HNO3 are needed to completely
react with 4.56 grams Cu?
Mass – Mass Calculations
g A  mol A  mol B
 gB
2 Al(OH)3  Al2O3 + 3 H2O
How many grams of H2O are produced when 250.0 grams
of Al(OH)3 are decomposed?
How many grams of Al2O3 are produced by the same
reaction?
Percentage Yield
% yield =
actual yield x 100
theoretical yield
Actual yield (“Experimental Yield”)– the amount of
product collected from an experiment.
Theoretical yield – the amount of product that is
expected to be produced.
Ex:
1. A chemical reaction is performed in which 115 g salt can
be produced. If 100. g is collected, what is the percent
yield of the experiment?
2. If a reaction has a theoretical yield of 355 g, but a
percentage yield of 68.5%, what will be the actual
amount of product collected?
3. Determine the percent yield of the reaction below if
55.0 g P is reacted and 110. g P4O10 is produced.
4 P + 5 O2  P4O10
Limiting Reactants
Definition:
the reactant that runs out first in a chemical
reaction; causes the reaction to stop; limits the
amount of product made.
Example:
To build a car….
1 car body
+
4 tires

1 car
If you had 1 car body+ 6 tires….
How many cars could you make? _____
The __________________is the limiting reactant. You
will use up the car body and you will still only be able
to make 1 car (according to the recipe).
The __________________ are the excess reactant. 4
of the tires would be used up, but you still have 2 tires
left over.
To make a peanut butter and jelly sandwich…
2 slices of bread + 2 Tbsp PB + 1 Tbsp. jelly
 1 PB&J sandwich
If you had a whole jar of PB and a whole jar of jelly,
but only 1 slice of bread….
How many PB&J sandwiches could you make? ____
What’s the limiting reactant? ________
The same idea can be applied to a chemical reaction:
2 H2
+ O2  2 H2O
If you had 3 moles H2 + 1 mole O2,
The __ is in excess. The __ is the limiting reactant.
If you had 2 moles H2 + 3 moles O2 ,
The __ is in excess. The __ is the limiting reactant.
If you had 4 moles H2 + 2 moles O2,
Both reactants will be used up completely. Nothing is
left over. You will make double the product.
Calculating Limiting and Excess Reactants
The easiest way to determine the limiting reactant
of a chemical reaction is to calculate the mass of
the product produced.
The reactant that produces the least amount of
product is the limiting reactant.
a)
of
b)
c)
d)
How many grams of Fe2S3 can be produced from the reaction
15.0 g Fe and 12.0 g S ?
Which reactant is the Limiting Reactant?
Which reactant is in excess?
How many grams of the excess reactant will be left over?