The molar mass constant, usually denoted by Mu, is a physical constant defined as one twelfth of the molar mass of carbon-12: Mu = M(12C)/12.[1] The molar mass of an element or compound is its relative atomic mass (atomic weight) or relative molecular mass (molecular weight or formula weight) multiplied by the molar mass constant.
The mole and the atomic mass unit (dalton) were originally defined in the International System of Units (SI) in such a way that the constant was exactly 1 g/mol, which made the numerical value of the molar mass of a substance, in grams per mole, equal to the average mass of its constituent particles (atoms, molecules, or formula units) relative to the atomic mass constant, mu. Thus, for example, the average molecular mass of water is approximately 18.015 daltons, making the mass of one mole of water approximately 18.015 grams.
On 20 May 2019, the SI definition of mole changed in such a way that the molar mass constant remains nearly but no longer exactly 1 g/mol. However, the difference is insignificant for all practical purposes. According to the SI, the value of Mu now depends on the mass of one atom of carbon-12, which must be determined experimentally. The 2022 CODATA recommended value of Mu is 1.00000000105(31)×10−3 kg⋅mol−1.[2][3]
The molar mass constant is important in writing dimensionally correct equations.[4] While one may informally say "the molar mass M of an element is the same as its atomic weight A", the atomic weight (relative atomic mass) A is a dimensionless quantity, whereas the molar mass M has the units of mass per mole. Formally, M is A times the molar mass constant Mu.