Percent composition can be calculated if the chemical formula is known. By using the molar masses of the elements (found on the periodic table), the molar mass of the entire compound can be found. [Make sure to pay attention to subscripts when calculating molar mass!]

To find the percent composition of the elements by molar mass, just divide the molar mass of the entire compound by the molar mass of the individual compound.

This will reveal the amount of the individual element within that compound relative to the compound's other elements out of 100%. When there is only one unknown composition left, its percent composition can be found by subtracting the composition(s) of the other element(s) from 100%.