Mole

Summary: The unit for amount of substance. One mole contains 6.02 × 10²³ particles (Avogadro’s constant). Tags: igcse chemistry definition stoichiometry Created: 2026-07-14 Last Updated: 2026-07-14

The mole (symbol: mol) is the SI base unit for the amount of a substance, defined such that one mole contains exactly 6.02 x 10^23 specified elementary entities — atoms, molecules, ions, electrons, or formula units — a number known as Avogadro’s constant. The mass of one mole of any substance in grams is numerically equal to its relative atomic mass (A_r) or relative formula mass (M_r), a value called the molar mass with units of g/mol; for example, carbon has A_r = 12, so one mole of carbon atoms has a mass of 12 g. The mole underpins nearly every quantitative calculation in IGCSE chemistry: the three central equations — moles = mass / molar mass, moles = concentration x volume (dm^3), and moles = volume of gas (dm^3) / 24 (at RTP) — allow students to interconvert between mass, solution concentration, and gas volume. Stoichiometry, the mole ratios derived from balanced chemical equations, enables prediction of reactant and product quantities, and IGCSE candidates must be able to calculate limiting and excess reagents, percentage yields, and the formulae of hydrated salts from experimental data. A common exam error is confusing the mole with molecular mass or using the wrong units; always remembering that the mole is a count of particles — like a “chemical dozen” — helps keep the concept clear.

Key term — IGCSE Chemistry (0620/0971) glossary.