Mole (unit)

The mole (symbol: mol) is the unit used to measure the amount of substance, one of the seven base units of the International System of Units (SI). The mole is used to count very small particles of matter—such as atoms, molecules, and ions—by using a large, manageable number.

The mole was created to connect the measurable mass of a physical sample with the number of small particles that cannot be seen directly. Chemists use the mole to calculate the stoichiometric coefficients of chemical reactions: each reaction has a definite ratio of particles between the reactants and products.

Modern definition

In 2019, a revision of the SI definitions fixed the mole to a specific number called Avogadro's number. One mole contains exactly 6.02214076 × 10²³ particles. The particles can be atoms, molecules, ions, electrons, or other specified particles. Before 2019, the mole was defined as the number of atoms in 12 grams of carbon-12, based on a mass measurement.

Avogadro's constant and molar mass

Avogadro's constant (N_A) is the number of particles in one mole, now a fixed value. Molar mass is the mass of one mole of a substance, usually expressed in grams per mole (g/mol). For example, the molar mass of water is approximately 18 g/mol, so about 18 grams of water contains one mole of molecules.

Applications

The mole is used routinely in chemistry, biology, and industry. Stoichiometric calculations depend on converting between mass and moles to determine how much of a substance is needed for a reaction. Concentration is also commonly expressed in moles per liter (mol/L), known as molarity.