The molar mass of some substance is the mass in grams of one mole of that substance. This mass is given by the atomic weight of the chemical unit that makes up that substance in atomic mass units (amu). For example, gold has an atomic weight of 196.967 amu, so one mole of gold has a mass of 196.967 grams. For a substance that is composed of more than one kind of atom, one adds up the atomic weights of the individual atoms for the chemical unit that makes up that substance. Water is composed of two atoms of hydrogen and one of oxygen. The atomic weight of one atom of hydrogen is 1.008 amu, so that of two atoms is 2.016. The atomic weight of one atom of oxygen is 15.999, so the molar mass of water is 2.016 + 15.999 = 18.015 grams.
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