Molar mass

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Molar mass


Molar mass M is the mass of a chemical compound divided by its amount-of-substance measured in moles. It is defined as MB = m/nB, where m is the total mass of a sample of pure substance and nB is the amount of substance B given in moles. The definition applies to pure substance. The molar mass allows for converting between the mass of a substance and its amount for bulk quantities. It is calculated as the sum of standard atomic weights of all atoms that form one entity of the substance. The appropriate SI base units is kg·mol-1. However, for historical as well as usability reasons, g·mol-1 is almost always used instead.

Abbreviation: M [kg·mol-1]; [g·mol-1]

Related terms

The relative molecular mass is represented by the symbol Mr and defined as MR = mf/mu. It is the mass of an entity (this can be a molecule or a formula unit: a group of atoms by the way the chemical formula is written) divided by the unified atomic mass unit. Since mass is divided by mass, the relative molecular mass is dimensionless.

The relative molar mass as well as the molecular weight are alternative terms for the relative molecular mass according to IUPAC's Green Book or Gold Book. Despite this definition, the term "molecular weight" is widely used as a synonym for the molar mass.

The formula weight is another synonym of the molar mass that is not covered by the IUPAC definition. For historical reasons and for non-molecular compounds, such as ionic salts, it is still frequently used.


Bioblast linkReferenceYear
Bureau International des Poids et Mesures (2019) The International System of Units (SI). 9th edition:117-216 ISBN 978-92-822-2272-0.2019
Cohen ER, Cvitas T, Frey JG, Holmström B, Kuchitsu K, Marquardt R, Mills I, Pavese F, Quack M, Stohner J, Strauss HL, Takami M, Thor HL (2008) Quantities, Units and Symbols in Physical Chemistry. IUPAC Green Book 3rd Edition, 2nd Printing, IUPAC & RSC Publishing, Cambridge.2008
Gnaiger E et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. Bioenerg Commun 2020.1.



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