A compound contains 75% carbon and 25% hydrogen by mass. what is the empirical formula for this compound?

These percentages are most likely mass percentages. Therefore, determine the mass of each element in 100g of the compound.

75g C, 25g H.

Divide these masses by their molar mass to find the ratios of the moles of element in the compound. Divide these values by the smallest of the two/three to get the ratios.

6.25:25 C:H - - > 1:4

So the formulae are CH4 (methane).