Example:
In a combustion reaction, a hydrocarbon (a compound that contains carbon and hydrogen and possibly oxygen) always reacts with oxygen to produce carbon dioxide and water.
Hydrocarbon + O2 ---> CO2 + H2O
C3H8 + 5O2 ---> 3CO2 +4H2O
Here is a trick to balance only combustion reactions:
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Balance the carbon
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Balance the hydrogen
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Count the number of oxygen atoms on the right
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Take half that number and put in front of oxygen on the left
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If you now have a fraction or decimal in front of O2, multiply all coefficients in front of all compounds by 2.