The mole
Cambridge A-Level Physics (9702) · Unit 15: Ideal gases · 6 flashcards
The mole is topic 15.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 15 — Ideal gases , alongside Equation of state and Kinetic theory of gases. In one line: The SI base unit for the amount of substance is the 'mole', symbol 'mol'. It quantifies the number of elementary entities (atoms, molecules, ions, etc.) of a substance.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 6 flashcards — 3 definitions, 1 key concept and 2 calculations — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The SI base unit for amount of substance
The SI base unit for the amount of substance is the 'mole', symbol 'mol'. It quantifies the number of elementary entities (atoms, molecules, ions, etc.) of a substance.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand that the SI base quantities include amount of substance, whose base unit is the mol.
- Use molar quantities, where a mole of a substance contains NA of its particles (NA being the Avogadro constant).
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
What is the SI base unit for amount of substance?
The SI base unit for the amount of substance is the 'mole', symbol 'mol'. It quantifies the number of elementary entities (atoms, molecules, ions, etc.) of a substance.
Define one mole in terms of the number of particles.
One mole of any substance contains exactly Nₐ particles (atoms, molecules, ions, etc.), where Nₐ is the Avogadro constant, 6.02 × 10²³ mol⁻¹. (This is very close to the number of atoms in 12 g of carbon-12, the older definition.)
What is the approximate value of the Avogadro constant, and what are its units?
The Avogadro constant (Nₐ) is approximately 6.022 × 10²³ mol⁻¹. It represents the number of particles (atoms, molecules, ions, etc.) per mole of a substance.
How do you calculate the number of moles (n) of a substance given the number of particles (N) and the Avogadro constant (Nₐ)?
The number of moles (n) can be calculated using the formula: n = N / Nₐ, where N is the number of particles and Nₐ is the Avogadro constant.
A sample contains 3.011 × 10²³ molecules of water (H₂O). How many moles of water are present?
n = N / Nₐ = (3.011 × 10²³) / (6.022 × 10²³) = 0.5 mol. Therefore, there are 0.5 moles of water.
Describe how the concept of the mole is important in stoichiometry.
The mole allows us to relate macroscopic measurements of mass to the number of atoms/molecules involved in a chemical reaction. Stoichiometric calculations rely on the mole ratio from balanced equations to predict quantities of reactants and products.
Review the material
Read full revision notes on The mole — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 15 — Ideal gases
The mole sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this The mole deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this The mole deck
Start in Study Mode, attempt each card before flipping, then rate Hard, Okay or Easy. Cards you rate Hard come back within a day; cards you rate Easy push out to weeks. Your progress is saved in your browser, so come back daily for 5–10 minute reviews until every card reads Mastered.
Study Mode
Rate each card Hard, Okay, or Easy after flipping.