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Molecular mass
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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, relative to the molecular mass unit u (equal to 1/12 the mass of one atom of carbon-12). Thus it is a dimensionless number.

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 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 18.01508 grams.

Molecular mass and relativity

Strictly speaking, the molecular mass is not precisely equal to the sum of the atomic masses, but slightly lower. This is because of the energy released upon the formation of chemical bonds, which is equivalent to "lost" mass according to special relativity. This difference is negligible however.

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