The aspect of chemistry that deals with the quantitative
relationships among reactants and products is called stoichiometry. The mole
concept is used for solving problems in stoichiometry.
Balanced equations are very important because they provide
the mole ratios. These ratios can be used to convert quantities of reactants or
products to moles, grams, or number of molecules.
Note the ratios of the substances in the synthesis of water:
2H2 + O2 -> 2H2O
This equation indicates that for every two moles of hydrogen
(H2), there is one mole of oxygen (O2) and 2 moles of
water (H2O). Therefore, the molar ratio of hydrogen to oxygen to
water is 2:1:2.
When doing a stoichiometry problem, the number of moles of
the starting material is converted to the number of moles of the desired substance,
using the mole ratio.
How many moles of oxygen are needed to react with 8 moles of
hydrogen?
The ratio of hydrogen to oxygen as shown in the equation is
2:1. Therefore, the amount of oxygen needed to react with 8 moles of hydrogen
is 4 moles of oxygen.
How many moles of hydrogen and oxygen are needed to produce
8 moles of water?
The ratio of hydrogen to water as shown in the equation is
1:1. Therefore, 8 moles of hydrogen can produce 8 moles of water. The ratio of
oxygen to water is 1:2. Therefore, 4 moles of hydrogen are needed to produce 8
moles of water.
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