Definition · Plain-language
The mole
The mole is the SI unit for amount of substance: one mole contains a fixed, very large number of particles — Avogadro’s number.
The step most authors miss
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A chemist’s counting unit
Atoms and molecules are far too small and numerous to count individually, so chemists use the mole as a counting unit, in the same way that "a dozen" means twelve. One mole is defined to contain exactly 6.022 × 10²³ elementary entities — Avogadro’s number. Whether those entities are atoms, molecules, ions or electrons, a mole is always that same enormous count. This gives chemists a consistent way to talk about "amount of substance", which is one of the seven SI base quantities, with the mole as its base unit.
Avogadro’s number and weighing by the mole
The mole’s real power is that it links the count of particles to a mass you can weigh. One mole of a substance has a mass in grams numerically equal to its molar mass — the sum of the relative atomic masses in its formula. One mole of carbon-12 weighs 12 grams; one mole of water weighs about 18 grams. So by weighing out 18 grams of water, a chemist knows they have about 6.022 × 10²³ water molecules, without ever counting them. This is the bridge between the laboratory balance and the world of atoms.
Using moles in reactions
Balanced chemical equations are really statements about moles. The coefficients tell you the ratio in which substances react and form — for example, 2H₂ + O₂ → 2H₂O means two moles of hydrogen react with one mole of oxygen to give two moles of water. By converting masses to moles using molar mass, applying these ratios, and converting back, a chemist can predict exactly how much product a reaction will yield or how much reactant is needed. This mole-based calculation is the heart of stoichiometry.
Key facts
At a glance
- Definition: the SI unit for amount of substance
- Contains: exactly 6.022 × 10²³ particles (Avogadro’s number)
- Applies to: atoms, molecules, ions or any specified entity
- Link to mass: one mole weighs its molar mass in grams
- Analogy: like a "dozen", but a fixed, very large count
- Used for: converting between mass, particles and reaction amounts
Common misconceptions
What people often get wrong
Often heard: A mole is a unit of mass, like the gram.
Actually: A mole is a unit of amount of substance — a count of particles — not a mass. The mass of one mole depends on the substance and equals its molar mass in grams.
Often heard: One mole of any substance has the same mass.
Actually: One mole always contains the same number of particles, but the mass differs by substance because the particles have different masses. A mole of carbon weighs about 12 g; a mole of water about 18 g.
Often heard: Avogadro’s number is an approximate, rounded estimate.
Actually: Since the 2019 SI redefinition, the mole is defined so that Avogadro’s number is an exact, fixed value of 6.022 140 76 × 10²³ entities per mole.
Going deeper
