Stoichiometry, Moles, and Molecular Weight

An important consideration when using the term mole is to specify the formula of the unit to avoid any misunderstanding.  For example, a mole of O contains 6.02 x 10²³ O atoms, but a mole of atmospheric oxygen, O₂, contains 12.04 x 10²³ oxygen atoms.

The molecular mass of something is the mass of one mole of that substance.  Carbon-12 has a molar mass of exactly twelve grams, by definition.  For all substances, the molar mass in grams is numerically equal to the formula weight in atomic mass units.  Sometimes, the molar mass is referred to as the gram formula or gram molecular weight.