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Quantities in Chemical
Reactions
STOICHIOMETRY
PROBLEMS
Section 5.4
What is Stoichiometry?
• The study of quantitative relationships
within chemical reactions
• A balanced equation is the key to
stoichiometry!
• Tools you’ll need for this chapter:
– Writing proper formulas and balanced equations
– Finding molar mass
– Converting from mass to moles and vice versa
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.
Stoichiometry Problems
• There are three types of stoichiometry
problems we will encounter:
– Mole-Mole problems (1 conversion factor)
– Mass-Mole problems (2 conversion factors)
– Mass-Mass problems (3 conversion factors)
given
required
Mole-Mole Problems
• Step 1: Write a BALANCED EQUATION
• Step 2: Determine the mole ratio from the
coefficients in the equation.
– Mole ratio = moles of required substance
moles of given substance
• Step 3: Multiply the amount of moles of the given
substance by the mole ratio
Mole-Mole Problems
Example:
2 H2 +
2 H2O
O2
How many moles of water can be formed from 0.5 mol
H2?
2 mol H O
0.5 mol H2 x
2
2 mol H2
= 0.5 mol H2O
Mole-Mole Practice
3 CuSO4 + 2 Al
Al2(SO4)3 + 3 Cu
How many moles of copper(II) sulfate will react with 0.5 moles of
aluminum?
Mole ratio
0.5 mol Al
x
3 mol CuSO4
2 mol Al
= 0.8 mol CuSO4
Mass-Mole Problems
Example:
2 H2 +
O2
2 H2 O
How many moles of water can be formed from 48.0 g O2?
Setting up the given information
2 H2 +
2 H2O
O2
Mass
Mass
48.0g
Moles
Mole Ratio
Moles
?
Mass-Mole Problems
• Step 1: Write a BALANCED EQUATION.
• Step 2: Convert the mass of your given substance
to moles using molar mass.
• Step 3: Determine the moles of your required
substance using the mole ratio.
Setting up the given information
2 H2 +
2 H2O
O2
Mass
Mass
48.0g
Step 2:
M = 32.00g/mol
Moles
Mole Ratio
Step 3
Moles
?
Step 1
Mass-Mole Problems
Example:
2 H2 +
2 H2 O
O2
How many moles of water can be formed from 48.0 g O2?
48.0 g O2 x
1 mol O2
32.00 g O2
x
2 mol H2O
1 mol O2
= 3.00 mol H2O
Mass-Mole Practice
How many moles of aluminum sulphate can be produced from
13.5 g of aluminum?
3 CuSO4 + 2 Al
Al2(SO4)3 + 3 Cu
Mole ratio
13.5 g Al x 1 mol Al
26.98 g Al
x 1 mol Al2(SO4)3 = 0.250 mol
Al2(SO4)3
2 mol Al
Mass-Mole Practice
3 Ca + 2 AlCl3
3 CaCl2 + 2 Al
How many moles of calcium chloride will be produced if 5.7g
of calcium is used up in the reaction?
Mass-Mole Practice
3 Ca + 2 AlCl3
3 CaCl2 + 2 Al
How many moles of calcium chloride will be produced if 5.7g
of calcium is used up in the reaction?
5.7 g Ca x 1 mol Ca
40.08 g Ca
x
3 mol CaCl2
3 mol Ca
= 0.14 mol CaCl2
Mass-Mass Problems
Example:
2 H2 +
O2
2 H2 O
How many grams of water can be formed from 48.0 g O2?
Setting up the given information
2 H2 +
2 H2O
O2
Mass
?
Mass
48.0g
Step 2:
M = 32.00g/mol
Moles
Step 1
Step 4:
M = 18.02g/mol
Mole Ratio
Step 3
Moles
Mass-Mass Problems
Example:
2 H2 +
O2
2 H2 O
How many grams of water can be formed from 48.0 g O2?
48.0 g O2 x 1 mol O2 x 2 mol H2O x 18.02 g H2O = 54.1 g H2O
32.00 g O2
1 mol O2
1 mol H2O
Mass-Mass Practice
3 Ca + 2 AlCl3
3 CaCl2 + 2 Al
How much aluminum is produced (in grams) when 1.9g of
calcium reacts with aluminum chloride?
Mass-Mass Practice
3 Ca + 2 AlCl3
3 CaCl2 + 2 Al
How much aluminum is produced (in grams) when 1.9g of
calcium reacts with aluminum chloride?
1.9 g Ca x
1 mol Ca
40.08 g Ca
x
2 mol Al
3 mol Ca
x 26.98 g Al
1 mol Al
0.85 g Al
=