A compound is 40.0% C, 6.7% H, and 53.3% O by mass. Find its empirical formula.

Assume a 100 g sample: 40.0 g C, 6.7 g H, and 53.3 g O.
moles C = 40.0 g / 12.01 g/mol = 3.33 mol
moles H = 6.7 g / 1.008 g/mol = 6.65 mol
moles O = 53.3 g / 16.00 g/mol = 3.33 mol
Divide by the smallest amount: C:H:O = 1:2:1.
The empirical formula is CH2O.