# Explain why chemists use the mole as a unit when working with substances, and state the number of particles present in exactly one mole of any substance.

> IB DP Chemistry Standard Level (2023 syllabus) — S1.4 Counting particles by mass: The mole · Explain · 4 marks

> Atoms and molecules are too small to count individually, yet chemists routinely work with measurable quantities of substances in the laboratory.

## Mark scheme (4 marks)

1. Atoms/molecules are too small (or too numerous) to count individually, so a counting unit is needed that links the microscopic and macroscopic scales.
2. The mole allows chemists to relate the mass of a substance (measurable on a balance) to the number of particles it contains.
3. This also allows the stoichiometry of chemical reactions (mole ratios) to be applied directly to laboratory-scale masses.
4. One mole of any substance contains 6.02 × 10²³ particles (Avogadro's constant, Nₐ).

## Key terms

- [mole](https://www.gradenine.co.uk/glossary/mole)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-chemists-use-the-mole-a1e5f3d0) · Published by Druglandscape Ltd.