Amount of substance
Amount of substance (symbol: n) is a physical quantity, and one of the seven base quantities in the International System of Quantities (ISQ). It measures the number of "entities" (for example, atoms or molecules) when the number too large to be directly counted.
Definition
Unlike the other base quantities in the ISQ, amount of substance is not defined by ISO 31:1992.[1] The International Union for Pure and Applied Chemistry (IUPAC) adopted the following definition in 1969,[2] and it is still in use[3][4] (although varient (equivalent) wordings have also been used[5][6]):
Amount of substance is a quantity proportional to the number of specified elementary entities N in a sample. The proportionality constant is the same for all substances, and is the reciprocal of the Avogadro constant NA, so that amount of substance is defined by the equation n = N/NA.
The entities may be atoms, molecules, ions, electrons, other particles, or specified groups of particles.
The IUPAC 1969 definition is an accurate description of how chemists use the concept, but it has some shortcomings from a metrological point of view. Prior to 1969, there was no specific name for the quantity n, and the term "number of moles" was used instead: neither the name nor the definition of a physical quantity should imply a choice of unit of measurement, hence the change.[2] However the 1969 definition makes it impossible to define the Avogadro constant NA without either specifying the unit of measurement or falling into a circular definition with amount of substance.
The following revised definition has been proposed,[1] and received some support from the Consultative Committe for Amount of Substance (CCQM) of the International Bureau for Weights and Measures (BIPM):[4]
Amount of substance is a quantity that measures the size of an ensemble of entities. It is proportional to the number of specified entities and the constant of proportionality is the same for all substances. The entities may be atoms, molecules, ions, electrons, other particles, or specified groups of particles.
As amount of substance is no longer defined in terms of the Avogadro constant, NA can be defined as the reciprocal of the constant of proportionality between n and N in any system of units and without risk of circularity.
References
- ↑ 1.0 1.1 Milton, M. J. T.; Mills, I. M. Amount of substance and the proposed redefinition of the mole. Metrologia 2009, 46, 332–38. DOI: 10.1088/0026-1394/46/3/022.
- ↑ 2.0 2.1 McGlashan, M. L. Manual of symbols and terminology for physicochemical quantities and units. Pure Appl. Chem. 1970, 21 (1), 1–44. DOI: 10.1351/pac197021010001; Physico-Chemical Quantities and Units; Royal Institute of Chemistry: London, 1971.
- ↑ Quantities, Units and Symbols in Physical Chemistry, 3rd ed.; Royal Society of Chemistry: Cambridge; p 53. ISBN 0-85404-433-7.
- ↑ 4.0 4.1 Consultative Committee for Amount of Substance: metrology in chemistry (CCQM): Report of the 15th meeting (22–24 April 2009); International Bureau for Weights and Measures: Sèvres, France; p 7, <http://www.bipm.org/utils/common/pdf/CCQM15.pdf>.
- ↑ Quantities, Units and Symbols in Physical Chemistry, 2nd ed.; Blackwell Science: Oxford, 1993; p 46. ISBN 0-63203-5838, <http://old.iupac.org/publications/books/gbook/green_book_2ed.pdf>.
- ↑ amount of substance, n, <http://goldbook.iupac.org/A00297.html> (accessed 29 March 2010), Compendium of Chemical Terminology Internet edition; International Union of Pure and Applied Chemistry (IUPAC).
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