8.6.1 Amount of substance Quantity symbol: n (also v).
SI unit: mole (mol).
Definition: See Sec. A.7.
Notes: 1. Amount of substance is one of the seven base quantities upon which the SI is founded (see
Sec. 4.1 and Table 1).
2. In general, n(xB) = n(B) / x, where x is a number. Thus, for example, if the amount of substance of
H
2
SO
4
is 5 mol, the amount of substance of (1/3)H
2
SO
4
is 15 mol: n[(1/3)H
2
SO
4
] = 3n(H
2
SO
4
).
Example: The relative atomic mass of a fluorine atom is A r
(F) = 18.9984. The relative molecular mass
of a fluorine molecule may therefore be taken as M r
(F
2
) = 2A r
(F) = 37.9968. The molar mass
of F
2
is then M(F
2
) = 37.9968 × 10
−3
kg/mol = 37.9968 g/mol (see Sec. 8.6.4). The amount of
substance of, for example, 100 g of F
2
is then n(F
2
) = 100 g / (37.9968 g/mol) = 2.63 mol.
8.6.2 Mole fraction of B; amount-of-substance fraction of B Quantity symbol: x B
(also y B
).
SI unit: one (1) (amount-of-substance fraction is a quantity of
dimension one).
Definition: ratio of the amount of substance of B to the amount of substance of the mixture: x B
= n B
/n.
Guide for the Use of the International System of Units (SI) Table 12. Summary description of nine quantities that are quotients involving amount of substance,
volume, or mass
(a)
