The formula mass of a compound is usually larger than the mass of an element. This is because a compound is made up of two or more elements, so the formula mass includes the masses of all of the elements in the compound. For example, the formula mass of water (H2O) is 18.015 g/mol, while the mass of hydrogen (H) is 1.008 g/mol and the mass of oxygen (O) is 15.999 g/mol.