Calculating Amounts

A blalnced equation is the critical tool for calculating the amounts of reactants or products in any chemical reaction.  The relationships in a balanced equation are expressed as stoichiometrically equivalent molar ratios.  The term "stoichiometrically equivalent" means that a definite quantity of one substances reacts with a definite quantity of all of the other substances represented in the equation.  The number of moles of one substance in the equation is stoichiometrically equivalent to the number of moles any other substance in a given reaction.  You can use stoichiometrically equvalent molar ratios as conversion factors to determine how much of one substance forms from (or reacts with) another.  Let's look at the methane combustion reaction to practice this technique.
 

         CH₄ (g)  +  2O₂(g)  ---->    CO₂(g)   +  2 H₂O(l)

In this reaction:

• 1 mole of CH₄ is stiochiometrically equivalent to 2 moles of O₂
• 2 moles of O₂ is stoichiometrically equivalent to 1 mole of CO₂
• 2 moles of H₂O is stoichiometrically equivalent to 1 mole of methane, 1 mole of CO₂ , 2 moles of H₂0 and 2 moles of O₂

To solve this problem you can construct a conversion factor from the balanced equation as follows:

This ratio can be used to solve the problem of how much oxygen is consumed when 10 moles of methane are burned.

Here is a problem that uses this approach in for a more complicated calculation.