A teacher shows two samples to a class: one mole of copper atoms and one mole of sulfur atoms. A student claims that both samples contain the same mass. State whether the student is correct or incorrect, and explain why, referring to the mole and the Avogadro constant in your answer.

Cambridge International IGCSE Chemistry (0620) — 3.3 The mole and the Avogadro constant · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The student is incorrect.

One mole of any substance contains the same number of atoms, 6.02×10²³ (Avogadro constant). The mass of one mole equals the relative atomic mass expressed in grams – the molar mass. Copper has a relative atomic mass of about 64, so one mole of Cu weighs ≈64 g. Sulphur has a relative atomic mass of about 32, so one mole of S weighs ≈32 g. Therefore the two samples do not have the same mass.

Examiner tips

  • State the student is incorrect first. Use the phrase ‘one mole contains 6.02×10²³ atoms’. Give the molar masses of Cu and S. Explain the mass difference clearly.”]
  • common_mistakes
  • :
  • Saying the masses are the same. Forgetting to mention the Avogadro constant. Using the wrong molar masses (e.g., 63.5 g for Cu).

Mark scheme (4 marks)

  1. The student is incorrect.
  2. One mole of any substance contains the same number of particles (6.02 × 10²³), equal to the Avogadro constant.
  3. The mass of one mole equals the relative atomic mass expressed in grams (the molar mass).
  4. Copper has a different (relative atomic) mass to sulfur, so the two samples have different masses (copper: 64 g; sulfur: 32 g).

Key terms in this question

mole · Avogadro constant

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