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Molecular mass

The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance and is expressed in terms of the molecular 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 molecular mass of a substance is equal to the mass in grams of one mole of that substance; this follows from the definition of "mole" and Avogadro's number. Therefore,

1 u = 1 gram/mole.

For example: the atomic mass of hydrogen is 1.00784 and that of oxygen is 15.9994; therefore, the molecular mass of water with formula H2O is (2 × 1.00784) + 15.9994 = 18.01508. Therefore, one molecule of water weighs 18.01508 u, and one mole of water weighs 18.01598 grams.

### Nitpicking

Strictly speaking, the molecular mass is not precisely equal to the sum of the atomic masses, but a little bit lower. This is because of the energy of the chemical bonds, which is equivalent to mass according to special relativity. This difference is neglible however.