A 10.0 g sample of a gas contains 2.5 g of hydrogen and 7.5 g of carbon. What is its percent composition?

1. Divide the mass of each element by the total mass of the compound and multiply by 100 percent. 2. For hydrogen: 2.5 g / 10.0 g x 100 percent = 25 percent. 3. For carbon: 7.5 g / 10.0 g x 100 percent = 75 percent. 4. Swapping the two labels gives the 25 percent C option, so read which element each mass belongs to. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.2 Determining Empirical and Molecular Formulas_