Empirical formulas and % composition

Question

If 100 g of aspirin contains 60.00 g of C, 4.480 g of H, and 35.53 g of O, what’s the empirical formula?

Answer 1

Any time you are given % comp, you can simply assume you have 100 g of sample and then % becomes grams. Here you are actually given the grams (not % comp), so ...

60.00 g C x 1 mol C/12 g = 5 mol C

4.480 g H x 1 mol H/1 g = 4.480 mol H

35.53 g O x 1 mol O/16 g = 2.22 moles

Dividing all by the lowest value in order to get whole numbers we get...

5/2.22 = 2.25 mol C

4.48/2.22 = 2 mol H

2.22/2.22 = 1 mol O

Now multiply all by 4 to get whole numbers...

2.25 mol C x4 = 9 mol C

2 mol H x 4 = 8 mol H

1 mol O x 4 = 4 mol O

Empirical formula = C₉H₈O₄