Amount of substance

The amount of substance, n, of a sample or system is a physical quantity which is proportional to the number of elementary entities present. "Elementary entities" may be atoms, molecules, ions, electrons, or particles, the choice of which is dependent upon context and must be stated. Amount of substance is sometimes referred to as chemical amount or, incorrectly, as number of moles.

The SI unit for amount of substance is the mole (mol), which is defined as the amount of substance that has an equal number of elementary entities as there are atoms in 0.012 kg (or 12 g) of carbon-12. That number is the Avogadro constant, NA, which has a value of 6.02214179(30) mol&minus;1.

Terminology
When quoting an amount of substance, it is necessary to specify the entity involved (unless there is no risk of ambiguity). One mole of chlorine could refer either to chlorine atoms (as in 58.44 g of sodium chloride) or to chlorine molecules (as in 22.711 dm3 of chlorine gas at STP). The simplest way to avoid ambiguity is to replace the term "substance" by the name of the entity and/or to quote the empirical formula. For example:
 * amount of chloroform, CHCl3
 * amount of sodium, Na
 * amount of hydrogen (atoms), H
 * n(C2H4)

This can be considered to be a technical definition of the word "amount", a usage which is also found in the names of certain derived quantities (see below).

Derived quantities
When amount of substance enters into a derived quantity, it is usually as the denominator: such quantities are known as "molar quantities". For example, the quantity which describes the volume occupied by a given amount of substance is called the molar volume, while the quantity which describes the mass of a given amount of substance is the molar mass. Molar quantities are sometimes denoted by a subscript latin "m" in the symbol, e.g. Cp,m, molar heat capacity at constant pressure: the subscript may be omitted if there is no risk of ambiguity, as is often the case in pure chemistry.

The main derived quantity in which amount of substance enters into the numerator is amount of substance concentration, c. This name is often abbreviated to "amount concentration", except in clinical chemistry where "substance concentration" is the preferred term (to avoid any possible ambiguity with mass concentration). The name "molar concentration" is incorrect, if commonly used.