The mole · Grade 9
Chemists count particles in moles. One mole is 6.02 × 10²³ particles (the Avogadro constant), and one mole of a substance has a mass in grams equal to its relative formula mass, Mᵣ. So moles link the mass you weigh out to the number of particles that react: n = m ÷ M.
Key facts
- Aᵣ is the relative atomic mass of an element; Mᵣ is the sum of the Aᵣ of all the atoms in a formula.
- One mole contains 6.02 × 10²³ particles (atoms, molecules or ions): the Avogadro constant.
- The mass of one mole in grams equals the Mᵣ: one mole of H2O has a mass of 18 g.
- moles = mass ÷ Mᵣ (n = m ÷ M), so mass = moles × Mᵣ.
- number of particles = moles × 6.02 × 10²³.
- A number after a bracket multiplies everything inside it: Ca(OH)2 has 1 Ca, 2 O and 2 H.