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SECTION 9.1
Introduction to
Stoichiometry
Teacher Notes and Answers
SECTION 1 Introduction to Stoichiometry
1. Reaction stoichiometry is a description of the
mass relationships between reactants and
products in a chemical reaction.
Practice
A. 19.5 mol
Review
1. Stoichiometry is the branch of chemistry that
deals with the mass relationships of elements in
compounds and the mass relationships between
reactants and products in a chemical reaction.
2. A mole ratio relates the number of moles that
react for two substances in a chemical reaction.
It is used to convert moles of one substance into
moles of another substance.
2 mol HgO 2 mol Hg 2 mol HgO
2 mol Hg 2 mol HgO 1 mol ​O2​ ​
3a.​ __________ ​ , ​ __________ ​ , ​ __________ ​,
1 mol ​O2​ ​ _________
1 mol ​O2​ ​ _________
2 mol Hg
​ __________
 ​ , ​ 
 ​ , ​ 
 ​
2 mol HgO 2 mol Hg 1 mol ​O2​ ​
4 mol N​H​ ​ 4 mol N​H​ ​ 4 mol N​H​ ​
6 mol NO 5 mol ​N2​ ​ 6 mol ​H2​ ​O
3b.​ __________3 ​ , ​ __________3 ​, ​ __________3  ​ ,
6 mol NO  ​ , ​ 6_________
6 mol NO  ​ ,
mol NO ​, ​ __________
​ __________
4 mol N​H3​ ​ 5 mol ​N2​ ​ 6 mol ​H2​ ​O
5 mol ​N​2​ _________
5 mol ​N2​ ​ __________
5 mol ​N2​ ​
​ __________
 ​ , ​ 
 ​, ​ 
 ​ ,
4 mol N​H3​ ​ 6 mol NO 6 mol ​H2​ ​O
6 mol ​H2​ ​O 6__________
mol ​H​2​O 6__________
mol ​H2​ ​O
​ __________
 ​
 ​ , ​ 
 ​ , ​ 
4 mol N​H3​ ​ 6 mol NO 5 mol ​N2​ ​
4. The equation must be balanced. The coefficients
of the balanced equation are necessary to relate
the amount of one substance to another.
Stoichiometry
1
SECTION 9.1
Introduction to
Stoichiometry
Earlier you learned about composition stoichiometry, which
describes the mass relationships of elements in a compound.
Reaction stoichiometry describes the mass relationships
between the reactants and products in a c­ hemical reaction.
Reaction stoichiometry is based on the law of conservation of
mass. Mass is conserved in balanced chemical equations, so
reaction stoichiometry problems always start with balanced
chemical equations.
READING CHECK
Write the definition of reaction stoichiometry in your
own words.
1.
Ratios of substances in chemical reactions can be
used as conversion factors.
Reaction stoichiometry problems can be approached by looking
at what information is given and what is unknown. There are
four basic types of problems. Each type of problem requires
taking the amount or mass of one substance and converting it
to the amount or mass of another substance. These conversions
are done using two quantities that will be discussed later in this
section: the mole ratio and the molar mass.
1.
Both the given and unknown quantities are amounts in
moles. In this type of problem, you are given the amount
of a substance in moles and asked to calculate the amount
in moles of another substance in a reaction. The general
plan to solve this type of problem is shown below. Only one
conversion is necessary to solve this type of problem.
convert
2
amount of
given
substance (mol)
CH A P TER 9
into
amount of
unknown
substance (mol)
Key Terms
composition stoichiometry
reaction stoichiometry
mole ratio
2.
The given quantity is an amount in moles and the unknown
quantity is a mass in grams. In this type of problem, you
are given the amount in moles of one substance and
asked to calculate the mass of another substance in the
chemical reaction.
TIP
If you are asked to find a
mass, look at the units.
Most of the time the masses will be
in grams, but sometimes they will
be in larger units, such as kilograms.
The general plan to solve this type of problem is shown
below. Two conversions are necessary to solve this type
of problem.
convert
3.
amount of
given
substance (mol)
into
amount of
unknown
substance (mol)
into
mass of
unknown
substance (g)
The given quantity is a mass in grams and the unknown
quantity is an amount in moles. In this type of problem,
you are given the mass of one substance and asked to
calculate the amount in moles of another substance in the
chemical reaction.
The general plan to solve this type of problem is shown
below. Two conversions are also necessary to solve this type
of problem.
convert
4.
mass of
unknown
substance (g)
into
amount of
given
substance (mol)
into
amount of
unknown
substance (mol)
Both the given and unknown quantities are masses in
grams. In this type of problem, you are given the mass of
one substance and asked to calculate the mass of another
substance in the chemical reaction.
The general plan to solve this type of problem is shown
below. Three conversions are necessary to solve this type
of problem.
convert
mass of
given
substance (g)
into
amount of
given
substance (mol)
into
amount of
unknown
substance (mol)
into
mass of
unknown
substance (g)
Stoichiometry
3
Mole Ratio
A mole ratio is a conversion factor that compares the amounts
of any two substances involved in a chemical reaction. The
mole ratio of the substances is determined by the coefficients
in the balanced chemical equation.
Consider, for example, the chemical equation for the
­electrolysis of melted aluminum oxide to produce aluminum
and oxygen. The equation states that 3 mol of aluminum oxide
yield 4 mol of aluminum and 3 mol of oxygen gas.
Mole Ratios
l_ _ _ 2​_ _ _ _​​O​
_ _ _ _​ 2
_ _ _ _ _ _ _ mol
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _A​
_ _ _ _ _ _ _ _  
_ _  ​
_ _ _ _ _ _3_ _ _ _ or _​ _ _ _ _ _ _ _ _4
_ _ _ _ _ _ _ _mol
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Al
_ _  
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _​_
4 mol Al
2 mol A​l2​ ​​O​3
l_ _ _2​_ _ _ _​​O​
​_ _ _ _ _ _ _ _2​_ _ _ _​_ _ _ _ _ _ _
_ _​ 2
_ _ _ _ _ _ _ _mol
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _A​
_ _ _ _ _ _ _ _  
_ _ _ ​
_ _ _ _ _3_ _ _ ​ or _​ _ _ _ _ _ _ _ _3
_ _ _ _ _ _ _ mol
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _O
   ​
3 mol O
​ 2​ ​
_​ _4_ _ _ _ _ _ _ mol
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Al
_ _ _ _ _ _ _ _ _ _ _ _ ​
3 mol O
​ 2​ ​
2 mol A​l2​ ​​O​3​
or
​_ _ _ _ _ _ _ _ _2​_ _ _​
_​ 3
_ _ _ _ _ _ _ _mol
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _O
 ​
4 mol Al
2​Al​2​​O​3​(l) ⟶ 4Al(s) + 3​O2​ ​(g)
If you are given the amount in moles for one substance,
multiply by the appropriate mole ratio to find the moles of the
other ­substance. The appropriate mole ratio is one that causes
the units to cancel correctly. For example, to find the amount
of aluminum that can be produced from 13.0 mol of aluminum
oxide, the mole ratio needed is that of Al to A​l2​ ​​O​3​.
13.0 mol A​l​2​​O​3​× __
​  4 mol Al
   ​= 26.0 mol Al
2 mol A​l2​ ​​O​3​
PRACTICE
A.
How many moles of ​O2​ ​would 13.0 mol of A​l2​ ​​O​3​produce?
Molar Mass
Remember that the molar mass of a substance is the mass, in
grams, of one mole of that substance. The molar mass can be
determined using the periodic table. It is the same value as the
atomic mass of the substance, but given in grams. For the
decomposition of aluminum oxide shown above, the molar
mass of Al is 26.98 g/mol, the molar mass of O​2​is 32.00 g/mol,
and the molar mass of A​l2​ ​​O​3​is 101.96 g/mol.
Suppose you needed to find the mass of aluminum in grams
that is equivalent to 26.0 mol of aluminum. You would convert
moles to grams using the molar mass as shown below. In
Section 2, several examples of how to use mole ratios and
molar masses in stoichiometric calculations will be presented.
26.98 g Al
26.0 mol Al = 26.0 mol Al × ​ __ ​= 701 g Al
1 mol Al
4
CH A P TER 9
TIP
To find the molar mass of a
compound, add together
the molar masses of each atom in
the ­compound. Remember to
multiply by any subscripts.
The molar mass of A​l2​​O​3​is two
times the molar mass of Al plus
three times the molar mass of O.
SECTION 9.1 REVIEW
VOCABULARY
1. What is stoichiometry?
2. What is a mole ratio, and how is it used in stoichiometry?
REVIEW
3. For each equation, write all possible mole ratios.
a. 2HgO(s) → 2Hg(l) + O
​ 2​ ​(g)
b. 4N​H3​ ​(g) + 6NO(g) → 5​N2​ ​(g) + 6​H2​ ​O(l)
Critical Thinking
4. RELATING IDEAS What step must be performed before any stoichiometry
problem is solved? Explain.
Stoichiometry
5