GCSE Chemistry

GCSE Chemistry moles, without the panic

Last updated September 2026

Moles trip up more GCSE Chemistry students than almost any other topic, and it's rarely the idea that's hard — it's that it's the first proper maths inside a subject that's mostly facts up to this point. One relationship does most of the work. Here it is, plus a free revision game that covers moles alongside the rest of the syllabus.

The equation that does most of the work

A mole is just a counting unit, like "a dozen" but for 6.02 × 10²³ particles — that number is Avogadro's constant, and it only really shows up at higher tier. What matters every time is this: moles = mass ÷ Mr. Mr is the relative formula mass — add up the relative atomic masses of every atom in the formula, straight off the periodic table.

Take sodium hydroxide, NaOH. Na is 23, O is 16, H is 1, so Mr = 40. 4 g of NaOH is therefore 4 ÷ 40 = 0.1 mol. Rearrange the same equation and you can go the other way — mass = moles × Mr — which is exactly what balanced equations ask you to do next: use mole ratios from the equation to work out how much of something else reacts or forms.

• Ar is one element's relative atomic mass. Mr is a whole formula's — add the Ar values up.
• Balanced equations give mole ratios, not mass ratios — convert to moles first, always.
• Answers usually want three significant figures unless told otherwise, so don't round early.

Where the marks actually slip away

Concentration questions are the most common trap: concentration (mol/dm³) = moles ÷ volume, but volumes are almost always given in cm³, and forgetting to divide by 1000 to convert to dm³ is the single most repeated error on mark schemes. Percentage yield — actual yield ÷ theoretical yield × 100 — trips people up the same way, because the theoretical yield needs its own full mole calculation first. If the formula work itself still feels shaky, it's worth going back to atomic structure first — moles calculations lean on knowing what's actually in an atom.

Questions students actually ask

What's the difference between Ar and Mr? Ar is the relative atomic mass of a single element, read straight off the periodic table. Mr is the relative formula mass of a whole compound — add up the Ar of every atom in the formula.

Do I need a calculator for moles questions? On the calculator paper, yes — use it even for numbers that look simple, since GCSE examiners like awkward decimals precisely because they catch out students doing it in their head.

Why does my percentage yield answer come out over 100%? That's almost always a units or rounding slip earlier in the calculation — percentage yield can't exceed 100% in a GCSE question, so it's a sign to check the theoretical yield step again.