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Stoichiometry - Calculations from Chemical Equations

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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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