mol

The mole (symbol: mol) is the SI unit used to express the amount of substance. It is defined as the amount of substance that contains exactly Avogadro's number of specified elementary entities. As of the 2019 redefinition, Avogadro's constant NA is fixed at 6.02214076 × 10^23 per mole, so one mole contains that many entities. Historically, the mole was tied to the amount of substance contained in 0.012 kilograms of carbon-12, a relation now superseded by the fixed constant.

One mole of any substance contains Avogadro's number of entities, whether atoms, molecules, ions, or other elementary units. The mole provides a bridge between microscopic particles and macroscopic quantities, enabling quantitative chemistry. For a pure substance, the mass of one mole is its molar mass M, expressed in grams per mole (g/mol). The numerical value of M equals the relative atomic or molecular mass in unified atomic mass units.

Key equations relate amount of substance to other quantities: n = N / NA, where n is the amount in moles and N is the number of entities; m = n × M, where m is mass in grams; N = n × NA. The mass of one mole of carbon-12 is defined as exactly 12 g, which anchors practical mass measurements to the mole.

Common uses include stoichiometric calculations, preparation of solutions, and expressing concentration via molarity (M = n/V, moles per liter). For gases, the molar volume denotes the volume occupied by one mole under specified temperature and pressure, approximately 22.4 L per mole at standard temperature and pressure (0°C, 1 atm).