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5.2 Introduction to Stoichiometry
What is Stoichiometry?
Stoichiometry is the study of the quantities
involved in chemical reactions.
The word stoichiometry means
“measuring the elements”
Stoichiometry allows a chemist to predict
how much of a reactant reacts or how
much of a product forms in a chemical
reaction.
Balanced Reaction Equations
and the Mole Ratio
 Consider this balanced reaction equation:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
The coefficients of this reaction represent:
- The number of reacting PARTICLES or
- The number of reacting MOLES
These numbers are FIXED, the ratio of reacting
substances NEVER changes.
Balanced Reaction Equations
and the Mole Ratio
 We can interpret this reaction in 2 ways:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
1) Every time 4 atoms of Al react with 3 molecules
of O2, 2 molecules of Al2O3 form.
OR
2) Every time 4 moles of Al atoms react with 3
moles of O2 molecules, 2 moles of Al2O3
molecules form.
Using the Mole Ratio
 Remember that the coefficients from a balanced
reaction represent the ratio of the moles of
substances that react and form during a
chemical reaction.
These numbers are fixed - they do not change
 We can use these ratios to predict the amounts
of substances that react and form in a reaction
when any amount of a substance is reacted.
 The conversion factor we use to make these
predictions is called a MOLE RATIO.

Using the Mole Ratio
In this reaction:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
What is the minimum amount of O2 moles
that react with 1.20 moles of Al?
To solve this, create a mole ratio:
coefficient of unknown
moles of unknown = moles given 
coefficient of given
Using the Mole Ratio
 To solve, substitute in the values:
- moles given = 1.20 moles Al
- moles of unknown = ? moles O2
- coefficient of given = 4 Al
- coefficient of unknown = 3 O2
 The mole ratio becomes:
3 O2
3.60 mol O2
moles O2 = 1.20 moles Al 
=
= 0.900 mol O2
4 Al
4
 We predict that 0.900 mol of O2 react with 1.20
mol of Al.
Using the Mole Ratio
 We can also predict the “theoretical yield” - which is
defined as the amount of a product forms if all the
reactants are converted to products.
 What it the theoretical yield of Al2O3 when 1.20 moles of
Al reacts? Using the mole ratio:
2 Al2O3
2.40 mol Al2O3
moles Al2O3 = 1.20 moles Al 
=
= 0.600 mol Al2O3
4 Al
4
 We predict that 0.600 mol of Al2O3 should form, if all the
reactants are converted into products (the theoretical
yield)
Terminology of Stoichiometry
 Coefficients
- Numbers that represent the ratios of reacting moles in a
balanced chemical reaction.
 Mole Ratio
- A conversion factor that allows you to determine how
much of one substance reacts or forms in a chemical
reaction.
 Theoretical Yield
- The amount of a product that should form in a chemical
reaction, if all of the reactants are converted into
products.