7.1: Mole Ratios in
Chemical Equations
 The study of the mass and amount
relationships between reactants and products
in a chemical reaction.
PARTICLE RELATIONSHIPS IN
BALANCED CHEMICAL EQUATIONS
 Consider the following reaction:
N2 + 3 H2 → 2 NH3
 If you wanted to produce 20 molecules of NH3, how many N2
molecules would you need?
 How many molecules of H2 are required to react with 9
molecules of N2?
Mole Ratios in Chemical Equations
 A chemical equation is like a recipe. The chemical formulas
indicate the “ingredients” of the reaction. The coefficients tell
you the amount needed.
 For example, the synthesis of water:
2 H2(g)
+
O2(g)
→
2 H2O(l)
Shows that hydrogen and oxygen molecules combine in a 2:1
ratio and produce 2 molecules of water.
2 H2(g)
+
O2(g)
→
2 H2O(l)
2 molecules : 1 molecule : 2 molecules
Mole Ratios in Chemical Equations
 Mole Ratio – the ratio of the amount (in moles) of
chemicals in a chemical equation
 The coefficients in the balanced equation tell us the
mole ratio. In the water example, the mol ratio is
2 H2(g)
+
O2(g)
→
2 H2O(l)
2 mol H2 : 1 mol O2 : 2 mol H2O
 We use MOLE RATIOS to predict the amounts in
moles of other substances in a reaction.
STEPS FOR SOLVING
STOICHIOMETRIC PROBLEMS!
 Step 1: Write the BaLaNcEd chemical equation
 Step 2: If you are given the mass or number of
particles of a substance, convert it to number of
moles!
 Step 3: Calculate the number of moles of the
required substance based on the number of moles
of the given substance, using the MOLE
RATIO!
 Step 4: Convert the number of moles of the
required substance or number of particles, as
directed by the question!
Mole Ratios in Chemical Equations
 Example 1: How many moles of N2 are required if 5.6
moles of ammonia are produced?
Mole Ratios in Chemical Equations
 Example 2: A freshly exposed surface of aluminum
quickly reacts with oxygen to form a layer of aluminum
oxide. What amount (in mols) of oxygen is required to
react completely with 6.4 x 10-2 mol of aluminum?
7.2: mass relationships
in Chemical Equations
Mass Relationships in Chemical
Equations
 We know that there is a relationship between the mass
of a substance and the number of moles it contains (m =
n x M).
 The reactants and products are in a balanced chemical
equation are also related by fixed ratios.
 Therefore ….
…if you know the amount of one substance in a
chemical reaction, you can determine the
amount of ANY other substance in that reaction!
Mass Relationships in Chemical
Equations
 Example 1: When burning 2 g of magnesium, how much
oxygen is required to produce magnesium oxide?
Mass Relationships in Chemical
Equations
 Example 2: Potassium chlorate decomposes to give
potassium chloride and oxygen gas. If 2.5 g of potassium
chlorate decomposes, what mass of oxygen will be
produced?
 Cont’d
Mass Relationships in Chemical
Equations
 How many moles of potassium chloride will be produced?
Page 319 # 1-3, page 320 #1-4, 6-9
Page 323-4 # 1-3, page 325 # 1-3,5, 7-9