to find the percent composition of a certain component of a compound, one needs only to find the molar mass of said particular part of a compound and divide it by the total mass of the compound. for example; to find the percent composition of CO2, you need to find the molar mass of carbon, (12.01) and the molar mass of oxygen, (16.00). multiply the masses by number of atoms in the compound, (16.00×2) (12.01×1) and you can find the total mass (44.02) then divide the totla mass of oxygen in the compound by that number and you can find the percentage. (32/44.01) the percentage of oxygen in CO2 is.7271, or rounded and put into percentage, 73%. empirical formulas are just the opposites of percent composition, it gives the percentages as givens, from there, you use molar masses to find the compound. you simply divide the mass of the component and divide it by the molar mass of the same component, from there, divide each of the products by the smallest product. This will give you the number of certain atoms in the compound. From there you can determine the empirical formula.