A chemist has two samples: 0.5 mol of sodium chloride (NaCl) and 0.5 mol of magnesium (Mg). Describe what is meant by the term 'mole' and explain why, although both samples contain the same number of moles, they have different masses.

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

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

Model answer (4 marks)

A mole is the amount of substance that contains 6.02×10²³ particles.
Both samples contain 0.5×6.02×10²³ particles, so the same number of atoms or formula units.
The relative formula mass of NaCl (58.44 g mol⁻¹) is different from that of Mg (24.31 g mol⁻¹).
Because mass of one mole = relative mass in grams, 0.5 mol NaCl has 29.22 g while 0.5 mol Mg has 12.16 g, giving different masses.

Examiner tips

  • Use the definition of a mole with Avogadro’s number; link to particles. State the relative masses of each substance. Show the calculation of the masses. Explain that moles measure quantity, not mass.

Common mistakes

  • Confusing moles with mass units. Using the wrong relative mass (e.g., atomic mass of Na instead of NaCl). Failing to show that both samples contain the same number of particles.

Mark scheme (4 marks)

  1. A mole is the amount of substance that contains the Avogadro constant (6.02 × 10²³) number of particles
  2. The relative formula mass / relative atomic mass of the two substances is different
  3. Mass of one mole = relative formula/atomic mass expressed in grams, so different Mr values give different masses
  4. Both samples contain the same number of particles (0.5 × 6.02 × 10²³), confirming moles measure amount not mass

Key terms in this question

mole

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