Question

A compound that is composed of only carbon and hydrogen contains 80.0% C and 20.0% H by mass. What is the empirical formula of the compound?

Answer 1

Take a 100 g sample. That contains 80 g C and 20 g H. Convert to mols.
mols = g/molar mass or g/atomic mass
mols C = 80/12 = 6.67
mols H = 20/1 = 20
Now you want to find the ratio of these two elements to each other in small whole numbers with the smallest number being no less than 1. The easy way to do that is to divide everything by the smaller number.
C = 6.67/6.67 = 1.00 for C
H = 20/6.67 = 2.998 for H which rounds to 3.0
Empirical formula is CH3