A compound is 40.0% C, 6.71% H and 53.28% O by mass. What is its empirical formula?

1. Assume a 100 g sample, giving 40.0 g C, 6.71 g H and 53.28 g O. 2. Carbon gives 40.0 g / 12.01 g/mol = 3.331 mol. 3. Hydrogen gives 6.71 g / 1.008 g/mol = 6.657 mol. 4. Oxygen gives 53.28 g / 16.00 g/mol = 3.330 mol. 5. The smallest amount is 3.330 mol, so divide all three by it. 6. That gives 1.000 for carbon, 1.999 for hydrogen and 1.000 for oxygen. 7. Rounding to whole numbers gives the empirical formula CH2O. 8. The option C2H4O2 has the same ratio but is not reduced to the smallest whole numbers, so it is not the empirical formula. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.2 Determining Empirical and Molecular Formulas_