In chemistry, the molar mass (M) (sometimes called molecular weight or formula weight, but see related quantities for usage) of a chemical compound is defined as the ratio between the mass and the amount of substance (measured in moles) of any sample of the compound.

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 an intensive property of the substance, that does not depend on the size of the sample.

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.

Since 1971, SI defined the "amount of substance" as a separate dimension of measurement.

Until 2019, 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.

: Here, Mr is the relative molar mass, also called formula weight.

(for non-molecular compounds), are older terms for what is now more correctly called the relative molar mass (Mr).

This is distinct but related to the molar mass, which is a measure of the average molecular mass of all the molecules in a sample and is usually the more appropriate measure when dealing with macroscopic (weigh-able) quantities of a substance.

The standard atomic weight takes into account the isotopic distribution of the element in a given sample (usually assumed to be "normal").

[10] The term formula weight has a specific meaning when used in the context of DNA synthesis: whereas an individual phosphoramidite nucleobase to be added to a DNA polymer has protecting groups and has its molecular weight quoted including these groups, the amount of molecular weight that is ultimately added by this nucleobase to a DNA polymer is referred to as the nucleobase's formula weight (i.e., the molecular weight of this nucleobase within the DNA polymer, minus protecting groups).

This is adequate for almost all normal uses in chemistry: it is more precise than most chemical analyses, and exceeds the purity of most laboratory reagents.

If a more accurate value of the molar mass is required, it is necessary to determine the isotopic distribution of the sample in question, which may be different from the standard distribution used to calculate the standard atomic mass.

The isotopic distributions of the different elements in a sample are not necessarily independent of one another: for example, a sample which has been distilled will be enriched in the lighter isotopes of all the elements present.

A useful convention for normal laboratory work is to quote molar masses to two decimal places for all calculations.

They may be calculated from standard atomic masses, and are often listed in chemical catalogues and on safety data sheets (SDS).

All of the procedures rely on colligative properties, and any dissociation of the compound must be taken into account.

The measurement of molar mass by vapour density relies on the principle, first enunciated by Amedeo Avogadro, that equal volumes of gases under identical conditions contain equal numbers of particles.

This principle is included in the ideal gas equation: where n is the amount of substance.

The vapour density (ρ) is given by Combining these two equations gives an expression for the molar mass in terms of the vapour density for conditions of known pressure and temperature: The freezing point of a solution is lower than that of the pure solvent, and the freezing-point depression (ΔT) is directly proportional to the amount concentration for dilute solutions.