Molar mass constant
| Value | 0.00100000000105 kg/mol |
| Status | Measured: ± 3.10e-13 kg/mol (3.1e-10 relative) |
| Source | CODATA 2022 |
| Categories | ThermodynamicChemistry |
M_u in every molar mass unit
| gram per mole | 1 g/mol |
| kilogram per mole | 0.001 kg/mol |
Learning zone
Molar mass is the relative atomic or molecular mass times this constant: M = Ar · Mu. For the whole of the twentieth century Mu was exactly 1 g/mol by construction, because the mole was defined as the number of atoms in 12 g of carbon-12, so a species of relative mass 18.015 had a molar mass of 18.015 g/mol by definition.
The 2019 redefinition fixed the Avogadro constant instead, which cut the tie to carbon-12 and turned Mu into a measured quantity. It is now 1.000 000 001 05(31) × 10⁻³ kg/mol — different from 1 g/mol by about one part in a billion, which is a rounding error in any chemistry you will ever do, and a real effect in mass metrology. CODATA still recommends using 1 g/mol for ordinary work.