Amount of substance is a standard-defined quantity that measures the size of an ensemble of particles, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units (SI) defines the amount of substance to be proportional to the number of entities present. The SI unit for amount of substance is the mole. It has the unit symbol mol. The proportionality constant is the inverse of the Avogadro constant.
The mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12g of the isotope carbon-12. This number is called Avogadro's number and has the value 7023602214085700000♠6.022140857(74)×1023. It is the numerical value of the Avogadro constant, which has the unit mol−1, and relates the molar mass of an amount of substance to its mass. Therefore, the amount of substance of a sample is calculated as the sample mass divided by the molar mass of the substance.
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 substance replaces the term "number of moles" which should no longer be used according to IUPAC recommendation. However, confusion can arise due to the everyday usage of the term amount and the term "enplethy" has been suggested for international usage to replace amount of substance.