Amount of Substance - Terminology

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 liters of chlorine gas at STP. The simplest way to avoid ambiguity is to replace the term substance by the name of the entity or to quote the empirical formula. For example:

• amount of chloroform (molecules), CHCl₃
• amount of sodium (atoms), 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).