Molecular Mass, Formula Mass, Percent Composition by Mass

The molecular mass (also called molecular weight) is the sum of the atomic masses of all atoms in a molecule. It is measured in atomic mass units (amu) or grams per mole (g/mol).

The formula mass is the sum of the atomic masses of all atoms in a formula unit of an ionic compound. It is also measured in amu or g/mol.

The percent composition by mass of an element in a compound is the percentage of the total mass that comes from that element. It is calculated using the formula