STOICHIOMETRY
(stoicheion, meaning “element” and metry, meaning “to measure”)
Have you ever followed a recipe? Recipes are written so that the ingredients combine in just the right way to
produce a delicious treat. Balanced chemical equations are the recipes in chemistry. They tell us what reactants
produce a specific substance and in what relative quantity. Stoichiometry is the study of the amount of substances
consumed and produced in chemical reactions. It is a way or relating each component in the chemical equation to
the other components. Consider all of the information that a balanced chemical reaction can provide. For example,
aluminum reacts with oxygen to form aluminum oxide. We know the following:
4 Al (s)
4 atoms Al
4(6.02 x 1023) atoms Al
4 moles Al
+
+
+
+

2 Al2 O3

2 formula units Al2 O3
 2(6. 02 x 1023 ) formula units Al2 O3

2 moles Al2 O3
3O2 (g)
3 molecules O2
3(6.02 x 1023) molecules O2
3 moles O2
By applying what we’ve learned about moles and chemical reactions, we can use this information to convert
between moles, mass, and volume. You must always write a balanced equation. You must also find a strategy that
allows you to get to the units you are looking for (translation: DIMENSIONAL ANALYSIS!). There is not one
solution that works for all problems. The best way to excel at this is through practice.
To learn this technique, let’s begin with a simple, non-chemical example: a recipe for peanut butter and jelly
sandwiches.
2 slices bread + 1 tablespoon butter + 3 teaspoons jelly  1 sandwich
A typical question might be “how many sandwiches can be made from 24 slices of bread?” Obviously, the answer is
12, but how would you set that up in dimensional analysis?
24 slices bread
1 sandwich
--------------------------
x
1
-------------------------
=
12 sandwiches
2 slices bread
In a stoichiometry problem, you are not limited to comparing reactants to products.
How many teaspoons of jelly would be required to use 36 slices of bread?
36 slices bread
-------------------------
1
3 teaspoons jelly
x
-------------------------
= 54 teaspoons of jelly
2 slices bread
Working problems with moles and a balanced chemical equation is a very similar process. Remember to show all
work with units!!!
EXAMPLE 1:
How many moles of ammonia are produced when 0.600 moles of nitrogen reacts with hydrogen?
N2 + 3H2  2NH3
EXAMPLE 2:
The formation of aluminum oxide is from the constituent elements aluminum and oxygen. How
many moles of aluminum are needed to form 2.30 moles of aluminum oxide?
4Al + 3O2 2Al2O3
EXAMPLE 3:
How many moles of copper are produced when 1.75 moles of iron (III) sulfate are produced in the
single replacement reaction between iron metal and copper (II) sulfate?
2Fe + 3CuSO4  Fe2(SO4)3 + 3Cu
STOICHIOMETRY HOMEWORK
Solve the following problems. Show all work as directed with units, or no credit will be given.
1.
How many moles of oxygen are produced by the decomposition of 6.25 moles of potassium chlorate?
2KClO3  2KCl + 3O2
2.
How many moles of hydrogen are produced from the reaction of 3.86 moles of zinc with an excess of
hydrochloric acid? Zn + 2HCl  ZnCl2 + H2
3.
How many moles of oxygen are necessary to react completely with 4.50 moles of propane, C3H8?
C3H8 + 5O2  3CO2 + 4H20
4.
How many moles of potassium nitrate are produced when 2.15 moles of potassium phosphate react with excess
aluminum nitrate? K3PO4 + Al(NO3)3  3KNO3 + AlPO4
5.
How many moles of sodium chloride are produced when 7.23 moles of iron (II) chloride reacts with excess
sodium phosphate? 3FeCl2 + 2Na3PO4  Fe3(PO4)2 + 6NaCl
6.
How many moles of potassium are required to react with 2.50 moles of aluminum chloride to produce
potassium chloride? 3K + AlCl3  3KCl + Al
7.
How many moles of oxygen are required to react with 2.50 moles of iron metal to produce iron (III) oxide?
4Fe + 3O2  2Fe2O3
8.
How many moles of hydrofluoric acid are required to produce 5.20 moles of silicon tetrafluoride according to
the following reaction? SiO2 + 4HF  2H2O + SiF4
9.
How many moles of water are required to produce 6.75 moles of arsenic acid in the reaction with diarsenic
pentoxide? As2O5 + 3H20  2H3AsO4
10. How many moles of carbon dioxide are produced in the combustion of 17.0 moles of octane, C8H18?
2C8H18 + 25O2  16CO2 + 18H2O