Amount of substance is the number of atoms, molecules, ions, or other specified group of entities present in a substance. The amount of substance (n) of an element. for example, is proportional to the number of atoms present. For all entities, the constant of proportionality is the Avogadro constant (formerly known as Avogadro's number). The SI unit of amount of substance is the mole.

 

The amount of substance is not usually measured directly. Chemists far more frequently measure the mass (m) of a product or reactant and then calculate the amount of substance from the molar mass (M) of the compound, since amount of substance is the ratio mass/molar mass: n = m/M.