Back Molêre massa Afrikaans Masa molar AN كتلة مولية Arabic Molyar kütlə Azerbaijani مول جرمی AZB Малярная маса Byelorussian Малярная маса BE-X-OLD Моларна маса Bulgarian Molarna masa BS Massa molar Catalan
| Molar mass | |
|---|---|
 | |
Common symbols | M |
| SI unit | kg/mol |
Other units | g/mol |
| Dimension | M N−1 |
In chemistry, the molar mass (M) (sometimes called molecular weight or formula weight, but see related quantities for usage) of a chemical substance (element or compound) is defined as the ratio between the mass (m) and the amount of substance (n, measured in moles) of any sample of the substance: M = m/n.[1] The molar mass is a bulk, not molecular, property of a substance. The molar mass is an average of many instances of the element or 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.
For a bulk sample of a substance X, the molar mass, M(X), is appropriate for relating the mass of the sample, m, and the amount of the substance in the sample, n, using the equation: n = m/M(X).
If N is the number of entities in the sample, and m(X) is the mass of each entity of the substance (atomic mass, molecular mass, or formula mass), then the mass of the sample is m = N ⋅ m(X), and the amount of substance is n = N/NA = N ent, where ent is an atomic-scale unit of amount equal to one entity, so the molar mass is M(X) = m(X) ⋅ NA = m(X)/ent, expressed as the atomic-scale mass per entity.
Given the relative mass (atomic weight, molecular weight, or formula weight) Ar(X) of an entity of a substance X, the atomic mass is m(X) = Ar(X) Da, so the molar mass can be expressed in daltons per entity as M(X) = Ar(X) Da/ent. Thus, the molar mass of a substance X can be calculated as M(X) = Ar(X) ⋅ Mu, with the molar mass constant Mu equal to Da/ent, which (for all practical purposes) is equal to g/mol, as one mole contains an Avogadro number of entities, and was historically defined such that the Avogadro number is the number of daltons in a gram (g/Da).
The molecular mass (for molecular compounds) and formula mass (for non-molecular compounds, such as ionic salts) are commonly used as synonyms of molar mass, differing only in units (dalton vs. g/mol or kg/kmol); however, the most authoritative sources define it differently. The difference is that molecular mass is the mass of one specific particle or molecule (a microscopic quantity), while the molar mass is an average over many particles or molecules (a macroscopic quantity).
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 coherent unit of molar mass is kg/mol. However, for historical reasons, molar masses are almost always expressed with the unit g/mol (or equivalently in kg/kmol).
The mole was defined in such a way that the numerical value of the molar mass of a compound in g/mol, i.e. M(X)/(g/mol), was equal to the numerical value of the average mass of one entity (atom, molecule, formula unit) in Da, i.e. m(X)/Da = Ar(X), so that M(X) = Ar(X) g/mol. It was exactly equal before the redefinition of the mole in 2019, and is now only approximately equal, but equality may still be assumed with high accuracy. Thus, for example, the average mass of a molecule of water is about 18.0153 Da, 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 calculated using the relative atomic mass of the element. Thus, for example, the molar mass of iron is about 55.845 g/mol.
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. Thus, during that period, the molar mass of carbon-12 was exactly 12 g/mol. The dalton was (and still is) defined as +1/12 of the mass of a carbon-12 atom. Using the relationship M(12C) = m(12C) · NA, with M(12C) = 12 g/mol, and m(12C) = 12 Da, rearranging gives the following expression for the Avogadro constant: NA = (g/Da) mol−1, making the Avogadro number equal to the number of daltons in a gram, equivalent to the number of atoms in 12 grams of carbon-12 (as in the 1971 definition of the mole).
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 entities, such that NA = 6.02214076×1023 mol−1, but because the dalton is still defined in terms of the mass of the carbon-12 atom, the Avogadro number is no longer exactly equal to g/Da. The numerical value of the molar mass of a substance expressed in g/mol, that is the numerical value of the mass of this number of entities of the substance expressed in grams, thus is (for all practical purposes) still equal to the numerical value of the average mass of an entity of the substance expressed in daltons—(the relative discrepancy is only of order 10–9, i.e. within a part per billion).
- ^ International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 41. Electronic version.