Moles and massHigher tier
Almost every chemistry calculation at Higher tier runs through moles. Get this one solid and reacting masses, percentage yield, concentration and titration all become the same problem wearing different clothes.
What a mole actually is
A mole is a counting number, in the same way that a dozen is. A dozen is 12 of something; a mole is 6.02 × 1023 of something. That number is the Avogadro constant, and AQA gives it to you.
Chemists count in moles because atoms are far too small and numerous to count individually. One mole of any substance contains the same number of particles as one mole of any other substance — one mole of carbon atoms and one mole of carbon dioxide molecules both contain 6.02 × 1023 particles, even though they weigh very different amounts.
Here is the bit that makes the whole topic work: the mass of one mole of a substance in grams is numerically equal to its relative formula mass. The Mr of water is 18, so one mole of water weighs 18 g. The Mr of sodium hydroxide is 40, so one mole weighs 40 g. That is why the equation below is so short — the periodic table is quietly doing the counting for you.
How to do it — step by step
- Work out the relative formula mass (Mr) of the substance from the periodic table.
- moles = mass in grams ÷ Mr
- Going the other way: mass = moles × Mr
- Check the units — mass must be in grams, and the answer is in mol.
Worked example — mass to moles
moles = 20 ÷ 40 = 0.5 mol
Worked example — moles to mass
mass = 2.5 × 100 = 250 g
Notice it is the same equation both times. If you can only remember one form, remember moles = mass ÷ Mr and rearrange it — you do not need to memorise two.
Getting the Mr right — where the marks actually go
In most moles questions the mole arithmetic is one mark and the Mr is where people come unstuck. Two things to watch.
Brackets multiply everything inside them. For calcium hydroxide, Ca(OH)2, the little 2 applies to the oxygen and the hydrogen: Mr = 40 + 2 × (16 + 1) = 74, not 40 + 16 + 1.
Use the bigger number on the periodic table. The relative atomic mass is the larger of the two figures next to the symbol; the smaller one is the atomic number. Picking the wrong one is the single most common way a moles question goes wrong from the very first line.
Mistakes that cost marks
- Mass in kilograms. The equation needs grams. If a question gives you 1.5 kg, that is 1500 g before you do anything else.
- Dividing the wrong way round. Moles are almost always a smallish number, and mass is usually the bigger one — if your answer is in the hundreds, check you have not done Mr ÷ mass.
- Forgetting the unit. The answer is in mol. Marks are dropped for a bare number.
- Rounding too early. Carry the full value through a multi-step question and round only at the end.
- Using the atomic number instead of the Ar. See above — it is worth double-checking every time.
Now practise it — free
Science Street’s Calculation Climb takes you from a worked example, through a guided step-by-step version, to questions you do yourself — with fresh numbers every time, and a built-in calculator and periodic table. Then it gives you a real AQA exam question on the same skill. No sign-up needed.
Practise moles on Science Street →