The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12).
The molecular mass can be calculated as the sum of the atomic masses of all the atoms of any one molecule.
The molar mass of a substance is numerically equal to the molecular mass, but expressed in mass units per mole (e.g. grams per mole)
- For example: the atomic mass of hydrogen is 1.00784 u and that of oxygen is 15.9994 u; therefore, the molecular mass of water with formula H2O is (2 × 1.00784 u) + 15.9994 u = 18.01508 u. Therefore, one molecule of water weighs 18.01508 u, and one mole of water weighs 18.01508 grams.
Molecular mass or molar mass are used in stoichiometry calculations.
Since molecules are created by chemical reactions, not nuclear reactions, a molecule's molecular mass exactly equals the sum of the atomic masses of its constituent atoms.