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,
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 H_{2}O 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.
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.
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