The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76×10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, *N*_{A}, when expressed in mol^{−1}, and is called the Avogadro number.

The amount of substance, symbol *n*, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.

The amount of B, *n*(B), is proportional to the number of entities of B, *N*(B), with

$$n\mathrm{(}\text{B}\mathrm{)}={N}_{\text{A}}^{-1}\text{\hspace{0.17em}}N\mathrm{(}\text{B}\mathrm{)}$$(1)

The proportionality factor is a universal physical constant that is independent of the nature of the substance. The reciprocal of the proportionality factor, *N*_{A}, is the Avogadro constant, which is the same for all substances. The stipulated Avogadro number is the fixed numerical value of the Avogadro constant, *N*_{A}=6.022 140 76×10^{23} mol^{−1}, which is provided by the CODATA Task Group on Fundamental Constants [14].

The Avogadro constant has the SI unit mol^{−1} because amount of substance, *n*, is a base quantity with the SI unit mole and because the number of entities, being a number, is a quantity of dimension one. The amount of B, *n*(B), is also proportional to the mass of B, *m*(B), with

$$n\mathrm{(}\text{B}\mathrm{)}=M{\mathrm{(}\text{B}\mathrm{)}}^{-1}\text{\hspace{0.17em}}m\mathrm{(}\text{B}\mathrm{)}$$(2)

The proportionality factor is the reciprocal of the molar mass, *M*(B), which is a characteristic constant of the substance B.

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