Amount of substance

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Amount of substance


The amount of substance, n, is a base physical quantity, and the corresponding SI unit is the mole [mol]. Amount of substance (sometimes abbreviated as 'amount' or 'chemical amount') is proportional to the number of specified elementary entities, Ni of that substance i, and the universal proportionality constant is the reciprocal value of the Avogadro constant [1],

ni = Ni/NA

ni contained in a system can change due to internal and external transformations,

dni = dini + deni

In the absence of nuclear reactions, the amount of any atom is conserved, e.g., for carbon dinC = 0. This is different for chemical substances or ionic species which are produced or consumed during the advancement of a reaction, r,

Amount dn.png

A change in the amount of i, dni, in an open system is due to both the internal formation in chemical transformations, drni, and the external transfer, deni, across the system boundaries. dni is positive if i is formed as a product of the reaction within the system. deni is negative if i flows out of the system and appears as a product in the surroundings [2].

Abbreviation: n [mol]

Reference: Cohen 2008 IUPAC Green Book, Gnaiger 1993 Pure Appl Chem

Communicated by Gnaiger E (last update 2018-11-02)


  1. 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. - »Bioblast link«
  2. Gnaiger E (1993) Nonequilibrium thermodynamics of energy transformations. Pure Appl Chem 65:1983-2002. - »Bioblast link«

MitoPedia concepts: MiP concept, Ergodynamics 

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