A 7.68 g sample of a compound contains 2.45 g of potassium, K, 2.22 g of chlorine, Cl, and oxygen, O. Calculate the empirical formula. Insert subscripts as needed.

Answer 1

The question is a little ambiguous because the way it reads it sounds like there are 2.22 g of chlorine and 2.22 g of oxygen. However if you add up all the grams, they don't equal 7.68. So, it must be that there are 2.22 g of chlorine and it contains oxygen as well, but we have to find the grams of oxygen (see below).

Convert grams to moles:

moles K = 2.45 g K x 1 mol K / 39.1 g = 0.0627 moles K

moles Cl = 2.22 g Cl x 1 mol Cl / 35.5 g = 0.0627 moles Cl

moles O = 7.68 g - 2.45 g - 2.22 g = 3.01 g O x 1 mol O / 16 g = 0.188 moles O

Divide all by 0.0627 to try to get whole numbers:

K = 1 mol

Cl = 1 mol

O = 3 mol

Empirical formula = KClO₃