Percent composition is used when you want to find out how much of a specific substance in a formula there is. Figuring out percent composition is actually pretty simple (or at least as simple as you can get in chemistry). So, lets find the percent composition of all the components in the formula Copper(2) Sulfur Pentahydrate, which also could be written as CuSO4 times 5 X H20.
1. The first step is to find the weight of all the elements in the formula. You can find this information on a periodic table. The weights are: Cu = 63.5 amu. S = 32.1 amu. O = 16.00 amu (times 4 because its the subscript). H = 1.01 amu (times 2 because its the subscript, times 5 because its the coefficient. O = 16.00 amu (times 5 because of its the coefficient). You then add all of these amounts together. They equal a sum of 249.6 amu.
2. The second step is to divide the element you are trying to find the percent for, by the total mass of the compound. So, in my example if I was trying to find the amount of Sulfur, I would take how much Sulfur weighs (32.1 amu) and divide that by the weight of the whole compound (249.6 amu). 32.1/249.6 = .127. The final step is multiplying this by 100 in order to turn the decimal into a percentage. .127 X 100 = 12.7% of the compound is Sulfur.