Skip to content

3.5 The mole and Avogadro's constant

What a mole is, the Avogadro constant, and how to convert between mass, moles and numbers of particles using Mr and 6.02 × 10^23.

Higher only AQA Edexcel OCR 1 min read

The mole

Key terms

  • Mole: the amount of a substance that contains 6.02 × 1023 particles (atoms, molecules, ions or electrons). Its symbol is mol.
  • Avogadro constant: the number of particles in one mole, 6.02 × 1023 per mole.

Chemists count particles in moles, because atoms are far too small to count one by one.

The mass of one mole of a substance, in grams, is equal to its relative formula mass (M_\mathrm{r}) (or A_\mathrm{r} for an element).

  • 1 mole of carbon atoms (A_\mathrm{r} = 12) has a mass of 12 g.
  • 1 mole of water molecules (M_\mathrm{r} = 18) has a mass of 18 g.

Both contain 6.02 × 1023 particles.

Mass and moles

moles = mass (g) ÷ M_r
A triangle linking mass, moles and Mr, and a chain from mass to moles (divide by Mr) to number of particles (multiply by 6.02 × 10^23), with a worked example for 9.0 g of water
A triangle linking mass, moles and Mr, and a chain from mass to moles (divide by Mr) to number of particles (multiply by 6.02 × 1023), with a worked example for 9.0 g of water

Examples:

  • Moles in 88 g of CO2 (M_\mathrm{r} = 44): 88 ÷ 44 = 2.0 mol
  • Mass of 0.25 mol of CaCO3 (M_\mathrm{r} = 100): 0.25 × 100 = 25 g

Moles and particles

number of particles = moles × 6.02 × 10^23

Example: 0.5 mol of water contains 0.5 × 6.02 × 1023 = 3.01 × 1023 molecules.

Each water molecule contains 3 atoms, so 0.5 mol of water contains 3 × 3.01 × 1023 = 9.03 × 1023 atoms.

Test yourself on this lesson

5 exam questions · 7 marks · with mark schemes and model answers

Start questions

Found a mistake?