Who this is for: Cambridge IGCSE Chemistry (0620) students who want the mole and Avogadro constant — converting between mass, moles and particle number — to become reliable marks instead of a formula they apply in the wrong direction.
What query it owns: how to understand and revise the mole and the Avogadro constant in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the mole revision-guide angle, while Tutopiya’s The Mole and the Avogadro Constant subtopic page owns the learning resource and the free Mole quiz owns the practice.

The mole is the chemist’s counting unit — one mole of any substance contains the same number of particles. In Cambridge IGCSE Chemistry (0620), examiners test conversions between mass, moles and particle number using Mr and the Avogadro constant (6.02 × 10²³). This guide covers the syllabus definitions, the triangle method that prevents errors, and the question types that appear every year.

Key takeaways

• One mole contains 6.02 × 10²³ particles (atoms, molecules or ions).
• Amount of substance (n) in moles = mass (g) ÷ Mr.
• Number of particles = n × 6.02 × 10²³.
• For gases at rtp, one mole occupies 24 dm³ (molar gas volume).
• Always identify what the particle is — atom, molecule or formula unit.

What is the mole in Cambridge IGCSE Chemistry?

The mole is the amount of substance containing 6.02 × 10²³ specified particles. The Avogadro constant links the number of particles to the amount in moles. One mole of any element has a mass in grams equal to its Ar; one mole of a compound has a mass equal to its Mr. This bridges the gap between invisible particles and measurable mass.

You can read the full explanation, conversion triangles and worked examples on Tutopiya’s The Mole and the Avogadro Constant subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Avogadro constant	6.02 × 10²³ particles per mole	”Calculate the number of atoms in 2 mol of carbon.”
n = mass / Mr	Convert grams to moles	”How many moles in 8 g of oxygen gas?”
Particles = n × L	L = 6.02 × 10²³	”How many molecules in 0.5 mol of water?”
Molar gas volume	24 dm³ per mole at rtp	”Calculate the volume of 2 mol of CO₂ at rtp.”
Molar ratio	Coefficients in balanced equation	”How many moles of H₂O from 3 mol of H₂?”

How to do mole calculations — step by step

1. Identify what you are counting — atoms, molecules, ions or formula units.
2. Calculate Mr if converting from mass.
3. Find moles: n = mass ÷ Mr (or n = volume ÷ 24 for gases at rtp).
4. Convert to particles if needed: multiply by 6.02 × 10²³.
5. Use molar ratio from the balanced equation for reacting amounts.
6. Convert back to mass or volume as the question requires.

Once you have worked through a few, test yourself with the free Mole quiz — it tells you fast whether the method has actually stuck.

Mass vs particles vs gas volume: which conversion does the question want?

Given	Find	Formula to use
Mass (g)	Moles	n = mass / Mr
Moles	Mass (g)	mass = n × Mr
Moles	Particles	particles = n × 6.02 × 10²³
Moles of gas at rtp	Volume (dm³)	volume = n × 24
Volume of gas at rtp	Moles	n = volume / 24

The mole in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical mole stem
Calculate	Show full conversion working	”Calculate the number of moles in 4 g of helium.”
Determine	Work out from experimental data	”Determine the mass of one mole of the compound.”
How many	Often particles or moles	”How many molecules are in 18 g of water?”
Show that	Prove a stated mole or mass value	”Show that 44 g of CO₂ is 1 mol.”
State	Give Avogadro constant or molar volume	”State the value of the Avogadro constant.”

Worked exam-style stems (how to answer the wording)

1. “Calculate the number of molecules in 9 g of water, H₂O.” Mr = 18; n = 9/18 = 0.5 mol; molecules = 0.5 × 6.02 × 10²³ = 3.01 × 10²³. Mark-scheme reward: Mr, moles, then particles.
2. “Calculate the volume occupied by 0.25 mol of carbon dioxide at rtp.” Volume = 0.25 × 24 = 6 dm³. Reward: molar gas volume applied correctly.
3. “2 g of magnesium reacts with hydrochloric acid. Mg + 2HCl → MgCl₂ + H₂. Calculate the volume of hydrogen at rtp.” n(Mg) = 2/24 = 0.0833 mol → n(H₂) = 0.0833 mol → volume = 0.0833 × 24 = 2 dm³. Reward: 1:1 ratio from equation + gas volume.

When you can recognise the wording instantly, work the full set on the Stoichiometry topical past paper questions and the Mole quiz to lock the method in.

How the mole connects to the rest of Stoichiometry

The mole depends on correct Formulae and Relative Masses. It feeds into titration, yield and concentration calculations across the syllabus. When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub lets you move straight from a weak subtopic into the next.

Common mistakes students make

• Using Mr of an atom when the question gives a diatomic gas (O₂, H₂, Cl₂).
• Multiplying by 6.02 × 10²³ when the answer should be in moles (or vice versa).
• Forgetting 24 dm³ applies only to gases at rtp.
• Confusing molecules with atoms — e.g. 1 mol H₂O has 2 mol of H atoms.
• Skipping the balanced equation ratio in multi-substance questions.

When you need more support

If mole calculations keep tripping you up — especially gas volume and particle number — work through the Stoichiometry topical past paper questions and the Mole quiz to pinpoint the exact gap, then get focused help from a Cambridge IGCSE Chemistry tutor to fix it quickly.

Frequently asked questions

Is the mole hard in Cambridge IGCSE Chemistry? No — the formulas are fixed. Marks are lost when students use the wrong Mr (especially diatomic gases) or confuse moles with particle number.

What is the Avogadro constant? 6.02 × 10²³ — the number of specified particles in one mole of any substance.

When do I use 24 dm³? For gases at room temperature and pressure (rtp): volume (dm³) = moles × 24.

How do I revise the mole effectively? Master Mr first, practise the n = mass/Mr triangle daily, then take the Mole quiz. Add gas volume and particle conversions once mass–mole is secure.

Ready to master Cambridge IGCSE Chemistry mole calculations?

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