How can I determine the empirical formula from percent composition?

To deduce the empirical formula of a compound, you need the ratio of the moles of each element, and you find that by using the percents of the element as the element’s mass.

As an example, if a compound is 16% Carbon and 84% sulfur, you can say that if you had a 100 gram sample of the compound, it would contain 16 grams of carbon & 84 grams of sulfur.

To find the moles of carbon in that sample, you would divide the mass by the of carbon, so 16/12 = 1.3 moles. You do the same calculation with the other . For Sulfur, you divide 84g by the atomic mass of sulfur, so 84/32 = 2.6moles of sulfur. You continue in this same way if there is more than 2 elements.

Finally you find the ratio of the moles of each element. The simplest way to do this is to find the element with the least number of moles and divide the other moles by that number. In the above example 2.6 moles of Sulfur divided by 1.3 moles of Carbon equals 2. (which is a 2:1 ratio) So there is twice as many sulfurs as carbons in this compound, and the empirical formula is CS2.