Explain how Avogadro's constant relates the macroscopic scale of measurement (grams) to the sub-microscopic scale (individual atoms or molecules), using the element carbon-12 as your reference point.

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).

Model answer (4 marks)

One mole of carbon‑12 has a mass of exactly 12 g, so the macroscopic mass of a sample can be linked directly to a defined amount of substance. Avogadro’s constant, Nₐ≈6.02×10²³ mol⁻¹, is defined as the number of atoms in that 12 g of carbon‑12. Thus Nₐ acts as a conversion factor between the number of individual atoms (sub‑microscopic) and the amount in moles (macroscopic). Because the relative atomic mass of any element numerically equals its molar mass in grams, the molar mass measured on a balance is the mass of one mole of that element’s atoms, linking the macroscopic scale to the sub‑microscopic scale.

Examiner tips

  • Use the exact 12 g definition of a mole of C‑12
  • State Nₐ≈6.02×10²³ mol⁻¹ as the conversion factor
  • Explain the link between relative atomic mass and molar mass
  • Keep the answer concise and use the term ‘mole’ correctly

Common mistakes

  • Confusing Avogadro’s number with the atomic mass of carbon‑12
  • Failing to mention the 12 g definition of a mole
  • Using the wrong value for Nₐ (e.g. 6.02×10²³ instead of 6.022×10²³)

Mark scheme (4 marks)

  1. One mole of carbon-12 has a mass of exactly 12 g (linking macroscopic mass to a defined amount of substance).
  2. Avogadro's constant (Nₐ ≈ 6.02 × 10²³ mol⁻¹) is defined as the number of atoms in exactly 12 g of carbon-12.
  3. Avogadro's constant acts as a conversion factor between the number of individual particles (sub-microscopic) and the amount in moles (macroscopic).
  4. Because the relative atomic mass of any element numerically equals its molar mass in grams, any element's molar mass can be measured on a balance (macroscopic) and directly corresponds to one mole of atoms (sub-microscopic).

Key terms in this question

Avogadro's constant · mole · carbon-12

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