Moles explained
What a mole is, the formula n = m ÷ Mr, and how to use it.
What is a mole?
A mole is just a way of counting particles, like "a dozen" means 12. One mole of anything contains 6.02 × 10²³ particles (atoms, molecules or ions). This number is called the Avogadro constant.
The key formula
moles = mass ÷ relative formula mass, or n = m ÷ Mr. Mass is in grams. Mr is found by adding up the relative atomic masses of every atom in the formula.
Finding Mr
For water, H₂O: there are 2 hydrogen atoms (Ar = 1) and 1 oxygen atom (Ar = 16), so Mr = (2 × 1) + 16 = 18.
✏️ Worked example
How many moles are in 36 g of water?
Mr of H₂O = 18. n = m ÷ Mr = 36 ÷ 18 = 2 moles.
🧠 Test yourself
How many moles are in 88 g of CO₂? (C = 12, O = 16)
Mr = 12 + 32 = 44, so n = 88 ÷ 44 = 2 mol.
What is the mass of 0.5 mol of NaCl? (Na = 23, Cl = 35.5)
Mr = 58.5, so m = 0.5 × 58.5 = 29.25 g.
How many atoms are in 1 mole of iron?
6.02 × 10²³ atoms.
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