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Molar mass constant

Mu=0.00100000000105 kg/molM_{\mathrm{u}} = 0.00100000000105\ \text{kg/mol}
Value1.00000000105 g/mol (0.00100000000105 kg/mol)
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StatusMeasured: ± 3.1 × 10⁻¹⁰ g/mol (3.1 × 10⁻¹⁰ relative), standard uncertainty
SourceCODATA 2022
CategoriesThermodynamicChemistry
Molar mass constant in every molar mass unit
UnitValue
gram per mole1 g/mol
kilogram per mole0.001 kg/mol
kilogram per kilomole1 kg/kmol
milligram per millimole1 mg/mmol
pound per pound-mole1 lb/lb-mol
dalton (as molar mass)1 Da
kilodalton (as molar mass)0.001 kDa

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.

Used by 2 solvers

Gas Density from Molar Mass

ρ=PMRT\rho = \frac{PM}{RT}

Ideal Gas Law

PV=nRTP V = n R T
Revised 16 October 2026
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formula.expert. (2026, October 16). Molar mass constant. https://formula.expert/constants/molar-mass-constant