in chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance in that sample, measured in moles. the molar mass is a bulk, not molecular, property of a substance. the molar mass is an average of many instances of the compound, which often vary in mass due to the presence of isotopes. most commonly, the molar mass is computed from the standard atomic weights and is thus a terrestrial average and a function of the relative abundance of the isotopes of the constituent atoms on earth. the molar mass is appropriate for converting between the mass of a substance and the amount of a substance for bulk quantities.
the molecular weight is very commonly used as a synonym of molar mass, particularly for molecular compounds; however, the most authoritative sources define it differently (see molecular mass).
the formula weight is a synonym of molar mass that is frequently used for non-molecular compounds, such as ionic salts.
the molar mass is an intensive property of the substance, that does not depend on the size of the sample. in the international system of units (si), the base unit of molar mass is kg/mol. however, for historical reasons, molar masses are almost always expressed in g/mol.
the mole was defined in such as way that the molar mass of a compound, in g/mol, is numerically equal (for all practical purposes) to the average mass of one molecule, in daltons. thus, for example, the average mass of a molecule of water is about 18.0153 daltons, and the molar mass of water is about 18.0153 g/mol.
for chemical elements without isolated molecules, such as carbon and metals, the molar mass is computed dividing by the number of moles of atoms instead. thus, for example, the molar mass of iron is about 55.845 g/mol.
between 1971 and 2019, si defined the "amount of substance" as a separate dimension of measurement, and the mole was defined as the amount of substance that has as many constituent particles as there are atoms in 12 grams of carbon-12. in that period, the molar mass of carbon-12 was thus exactly 12 g/mol, by definition. since 2019, a mole of any substance has been redefined in the si as the amount of that substance containing an exactly defined number of particles, n = 6.02214076×1023. therefore, the molar mass of a compound now is simply the mass of this number of molecules of the compound.