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2.2 The mole and the Avogadro constant

Syllabus
9701–2028–2029
Topic
2.2
Level
AS

One mole contains the Avogadro constant of specified entities

A mole is an amount of substance containing the Avogadro constant, L = 6.022 × 10²³ mol⁻¹, of specified entities. The entities may be atoms, molecules, ions, electrons or formula units, so always state what is being counted.

N=nL\mathrm{N = nL}

N is the number of entities, n is the amount in moles, and L is the Avogadro constant. For 0.250 mol of H₂O, N = 0.250 × 6.022 × 10²³ = 1.51 × 10²³ water molecules to three significant figures.

A chemical formula controls counts inside each entity: those water molecules contain twice as many H atoms, or 3.01 × 10²³ H atoms. One mole does not mean one mole of every atom shown in a formula.

The mole measures amount of substance, not mass or volume. Different one-mole samples contain the same number of specified entities but can have different masses and occupy different volumes.