Amount of substance

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Amount of substance

Description

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, NX of that entity X, and the universal proportionality constant is the reciprocal value of the Avogadro constant [1],

nX = NX/NA

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

dnX = dinX + denX

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 Xi, 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 Xi is formed as a product of the reaction within the system. deni is negative if Xi 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 2020-05-18)


References

  1. Bureau International des Poids et Mesures (2019) The International System of Units (SI). 9th edition:117-216 ISBN 978-92-822-2272-0. - »Open Access pdf«
  2. 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«
  3. Gnaiger E (1993) Nonequilibrium thermodynamics of energy transformations. Pure Appl Chem 65:1983-2002. - »Bioblast link«
  1. Gnaiger E (2019) Mitochondrial pathways and respiratory control. An introduction to OXPHOS analysis. 5th ed. Mitochondr Physiol Network 24.05. Oroboros MiPNet Publications, Innsbruck:112 pp. - »Bioblast link«

References

SI-units.png
Base quantity Symbol for quantity Symbol for dimension Name of SI unit Symbol for SI unit [*]
length l L meter m
mass m M kilogram kg
time t T second s
electric current I I ampere A
thermodynamic temperature T Θ kelvin K
amount of substance *,§ n = N·NA-1 N mole mol
count *,$ N X counting unit x
charge *,€ Q = N·e = n·F I·T coulomb C = A·s
luminous intensity Iv J candela cd
[*] »SI base units
* For the quantities n, N and Q, the entity X has to be specified in the text or indicated by a subscript or in parentheses: nX; NX; QX.
§ 'Amount is a counting quantity, converting the counting unit [x] into moles [mol] using the Avogadro constant, NA.
$ 'Count' is synonymous with 'number of counting entities'. In the SI, the quantity 'count' is explicitly considered as an exception: "Each of the seven base quantities used in the SI is regarded as having its own dimension. .. All other quantities, with the exception of counts, are derived quantities" (Bureau International des Poids et Mesures 2019 The International System of Units (SI)).
Charge is a derived SI quantity, included here for comparison. Charge is a counting quantity, converting the counting unit [x] into coulombs [C] using the elementary charge, e, or converting moles [mol] into coulombs [C] using the Faraday constant, F.


SI-units.png


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