Write the unbalanced reaction. For example, if the reactants are butane and oxygen, the products are carbon dioxide and water. The unbalanced equation is written as follows: C4H10 + O2 ' CO2 + H2O. The number of each atom on the left does not balance with the number on the right.
Count the number of atoms on the left side of the equation and write them down. In the example, there are 4 carbon, 10 hydrogen and 2 oxygen atoms on the left side of the equation.
Count the number of atoms on the right side of the equation and write them down. In the example, there are 1 carbon, 3 oxygen and 2 hydrogen atoms on the right side of the equation.
Find an element that is in only one species on both sides of the equation. In the example, carbon is only in butane on the left and CO2 on the right, so start with carbon. In any equation, it is best to save hydrogen and oxygen for last.
Multiply the species with the lower number by a coefficient so that it will equal the number of species with the higher number. In the example, multiply CO2 by 4 to get 4 carbon atoms on both sides. Thus, C4H10 + O2 ' (4)CO2 + H2O.
Repeat step 5 for the next unbalanced atom. Balance oxygen last, since it is likely to appear in the most species. In the example, there are 10 hydrogen atoms on the left and 2 on the right. Balance the hydrogen by multiplying H2O by 5, since H2 multiplied by 5 = 10 hydrogen atoms. Thus, C4H10 + O2 ' (4)CO2 + (5)H2O. There are now 10 hydrogen atoms on both sides of the equation.
Repeat step 5 for each remaining atom until all the atoms are balanced. If they are already balanced, proceed to step 9. If, however, you need to multiply by a fraction or mixed number to make atoms balance, first multiply every species on one side of the equation to get rid of the fraction and then balance the atoms as in step 5. In the example, there are now 2 oxygen atoms on the left and 13 of them on the right side of the equation. You need to multiply O2 by 6 1/2 to get 13, but you cannot have 1/2 an atom. To correct this, first multiply all species on the right by 2. Thus, C4H10 + O2 ' 2 [(4)CO2 + (5)H2O] = C4H10 + O2 ' (8)CO2 + (10)H2O. There are now 26 oxygen atoms on the right. Finish by multiplying O2 by 13, since O2 multiplied by 13 = 26 oxygen atoms. Thus, C4H10 + (13)O2 ' (8)CO2 + (10)H2O. There are now 26 oxygen atoms on both sides of the equation.
Re-balance affected atoms by repeating step 5. In the example, the carbon and hydrogen atoms are now unbalanced, since you multiplied the entire right side of the equation by 2. To correct this, multiply C4H10 by 2: (2)C4H10 + (13)O2 ' (8)CO2 + (10)H2O. There are now 8 carbon and 20 hydrogen atoms on both sides of the equation.
Check your work to ensure that all atoms balance. In the example, (2)C4H10 + (13)O2 ' (8)CO2 + (10)H2O. There are 8 carbon atoms on both sides, 20 hydrogen atoms on both sides and 26 oxygen atoms on both sides. The equation is balanced.