A 17.90 g sample of a compound contains 5.71 g of potassium, K, 5.18 g of chlorine, Cl, and oxygen, O. Calculate the empirical formula. Insert subscripts as needed.

Answer 1

To find the empirical formula, we want to first find the number of MOLES of each element that is present. To do this, we convert the grams of each element to moles:

moles K = 5.71 g x 1 mol K/39.098 g = 0.1460 moles K

moles Cl = 5.18 g x 1 mol Cl/35.453 g = 0.1461 moles Cl

moles O = 5.18 g x 1 mol O/15.999 g = 0.3238 moles O

So, now that we know the moles of each, those would be the subscripts for the empirical formula, and it would be K_0.1460Cl_0.1461O_0.3238, but as you know, we don't use fractions of moles, so we need to get these to be whole numbers. The easiest way to do this is to divide all by the smallest (0.1460) to get that to be 1.00. In doing so, we get the following:

K = 0.1460/0.1460 = 1.00

Cl = 0.1461/0.1460 = 1.00

O = 0.3238/0.1460 = 2.2 ≈ 2

So, the empirical formula becomes KClO₂