Moles Basics | GCSE Chemistry

Quantitative chemistry

Lesson overview

This lesson introduces the core chemistry idea, the useful equipment and the calculation or data skills used on this page.

Focusrelative formula mass, amount of substance and moles

Time45-60 minutes

EquipmentPeriodic table, calculator and notebook.

Practical linkaccurate measurement supports quantitative practical work

Maths tagsAr, Mr, moles and Avogadro-style proportional reasoning

What you will learn

Describe the key chemistry ideas behind relative formula mass and moles.
Use precise GCSE command-word language in explanations.
Apply the idea to unfamiliar substances, data or practical contexts.
Check answers using units, state symbols, observations or balanced equations where relevant.

Core knowledge

Relative formula mass, Mr, is found by adding the relative atomic masses of every atom in the formula.
Amount in moles links mass to particles: moles = mass / Mr.
Rearrange carefully: mass = moles x Mr.
Read formulae carefully, especially brackets and small numbers.
Mass is usually measured in grams and amount is measured in mol.

Quantitative chemistry infographic

Illustrated GCSE Chemistry infographic showing relative formula mass, moles, balanced equations, concentration and exam calculation routines. — Use this infographic to connect formula mass, mole calculations, balanced equations, concentration and clear calculation routines.Download visual

Moles Basics practice set

Use the worked examples and practice questions on this page as a complete study task: learn the relative formula mass and mole meanings, summarise the infographic in your own words, then answer the questions using the data, equations and observations given here. Check every answer for Ar, Mr, moles and Avogadro-style proportional reasoning.

Clear explanation

Moles are a counting unit for chemicals. Because atoms and molecules are too small to count directly, GCSE calculations use mass and Mr to find amount in mol.

The most common error is using the wrong Mr because the formula has been read too quickly. Count atoms before substituting into the equation.

Good calculation answers show equation, substitution, answer and unit.

Worked examples

Core example: CO2

Ar: C = 12, O = 16.

Mr of CO2 = 12 + 16 + 16 = 44.

moles = mass / Mr = 4.4 / 44 = 0.10 mol.

Answer: 4.4 g of CO2 is 0.10 mol.

Extension example: calcium carbonate

Mr of CaCO3 = 40 + 12 + (3 x 16) = 100.

mass = moles x Mr.

mass = 0.25 x 100 = 25 g.

Answer: 0.25 mol of CaCO3 has a mass of 25 g.

Quick checks

Choose an answer, then check your thinking.

1. What is the Mr of H2O? Ar: H = 1, O = 16.

17 34 18

2. Which equation calculates moles from mass?

moles = mass / Mr moles = mass x Mr moles = Mr / mass

Practice questions

Question 1

Calculate the Mr of MgO. Ar: Mg = 24, O = 16.

Reveal answer and marking guidance

Answer: Mr = 24 + 16 = 40.

Marking: Credit correct addition.

Question 2

Calculate the amount in mol in 10 g of CaCO3. Mr = 100.

Reveal answer and marking guidance

Answer: moles = 10 / 100 = 0.10 mol.

Marking: Credit equation, substitution and unit.

Question 3

Calculate the mass of 0.50 mol of NaCl. Mr = 58.5.

Reveal answer and marking guidance

Answer: mass = 0.50 x 58.5 = 29.25 g.

Marking: Credit correct rearrangement and grams.

Question 4

Stretch: Why does 1 mol of CO2 have a greater mass than 1 mol of H2O?

Reveal answer and marking guidance

Answer: They contain the same number of molecules, but CO2 has a greater Mr.

Marking: Credit same amount of particles and higher Mr.

Practice ladder

FluencyRecall the key definition, symbol, structure, equation or observation.

ApplicationApply relative formula mass and moles to unfamiliar substances, practicals or data.

Practical interpretationUse evidence, method quality, uncertainty or conclusion wording where asked to evaluate.

Maths skillUse units, ratios, graphs and significant figures accurately.

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. Marks come from using the correct chemistry model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as particles, ions, bonds, energy, rate, evidence and uncertainty.

Common mistakes

Forgetting atoms after brackets.
Writing moles without the unit mol.
Using atomic number instead of relative atomic mass.
Rounding too early.

Extension challenge

Create a six-line revision card for relative formula mass and moles: two definitions, one infographic detail or equation line, one equation or data check, one common mistake, and one complete explanation sentence using relative formula mass, mole, Avogadro.

Reveal answer

Example answer: A complete response names the chemistry model, uses accurate units or observations, and explains why the evidence supports the conclusion.