Acompound is 80.0% carbon and 20.0% hydrogen by mass. assume you have a 100.-g sample of this compound.

The empirical formula is:

Explanation:

In a sample of 100 g:

To determine the molecular formula you need to calculate the moles of each element. To do that the atomic weights are needed:

So, the moles of each one:

Now, you need to take this values to non-fractional numbers (keeping the ratio):

Therefore the empirical formula is:

The molecular formula probably is a multiple of the empirical due to the carbon's valence.