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ISO 31-8 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to physical chemistry and molecular physics.

Its definitions include:

Quantity			Unit		Remarks
Name	Symbol	Definition	Name	Symbol
relative atomic mass	Ar	Ratio of the average mass per atom of an element to 1/12 of the mass of an atom of the nuclide 12C	one	1	Formerly called atomic/molecular weight. Example: Ar(Cl) = 35.453. Both quantities depend on the nuclidic composition.
relative molecular mass	Mr	Ratio of the average mass per molecule or specified entity of a substance to 1/12 of the mass of an atom of the nuclide 12C
number of molecules or other elementary entities	N	Number of molecules or other elementary entities in a system	one	1
amount of substance	n, (ν)		mole	mol	The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kg of 12C. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specific groups of such particles. The definition applies to unbound atoms of 12C, at rest and in their ground state.
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