15-­‐10-­‐26 Chapter 7: Stoichiometry
7.1: Mole Ratios in Chemical
Equations
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)
2 molecules : 1 molecule : 2 molecules
¢  Shows
that hydrogen and oxygen molecules combine
in a 2:1 ratio and produce 2 molecules of water.
Mole Ratios in Chemical Equations
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 mol H2 : 1 mol O2 : 2 mol H2O
Steps:
¢  Step 1: Write a balanced chemical equation for the reaction,
listing the given value(s) and required value(s) below the
substances being considered in the problem. Use the symbol
of the required value since its value is unknown.
¢
¢  We
use MOLE RATIOS to predict the amounts in
moles of other substances in a reaction.
Example 1: What amount (in mols) of hydrogen is produced
when 5.2 mol of ammonia decomposes?
Step 2: Convert amount of the given substance into the
amount of the required substance. To do this, you multiply
the given substance by a conversion factor derived from the
MOLE RATIO in the balanced equation.
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?
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