Moles

Chemists count particles in moles (symbol mol). One mole of any substance contains 6.02 × 10²³ particles — atoms, molecules or ions. This number is the Avogadro constant.

The mass of one mole of a substance, in grams, equals its Aᵣ or Mᵣ:

• 1 mol of carbon (Aᵣ 12) has a mass of 12 g.
• 1 mol of water (Mᵣ 18) has a mass of 18 g.

Both contain 6.02 × 10²³ particles: carbon atoms in one case, water molecules in the other.

moles = mass (g) ÷ Mᵣ

Rearrange it to find mass: mass = moles × Mᵣ

Example 1: how many moles are in 22 g of carbon dioxide, CO₂? Mᵣ = 12 + (2 × 16) = 44.

moles = 22 ÷ 44 = 0.5 mol

Example 2: what is the mass of 2 mol of water (Mᵣ 18)?

mass = 2 × 18 = 36 g

To find the number of particles, multiply the number of moles by the Avogadro constant:

number of particles = moles × 6.02 × 10²³

Example: how many molecules are in 22 g of CO₂?

1. moles = 22 ÷ 44 = 0.5 mol
2. molecules = 0.5 × 6.02 × 10²³ = 3.01 × 10²³