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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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

Model answer (4 marks)

Chemists use the mole because atoms and molecules are far too small or too numerous to count individually; a counting unit is required to link the microscopic and macroscopic scales.
The mole provides a convenient way to relate the mass of a substance, which can be measured on a balance, to the number of particles it contains.
This relationship allows stoichiometric calculations to be carried out directly from laboratory‑scale masses.
One mole of any substance contains 6.02 × 10²³ particles (Avogadro’s constant).

Examiner tips

  • Use the word ‘mole’ and Avogadro’s constant explicitly; link mass to particle number; mention stoichiometry; keep answer concise and to the point.
  • Structure answer in four clear points matching the mark scheme.

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 in this question

mole

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