A student says: 'Two moles of hydrogen gas (H₂) and two moles of neon gas (Ne) must have the same mass because they contain the same number of particles.' Evaluate this statement, explaining whether the student is correct or incorrect about the number of particles and about the mass.

Cambridge International IGCSE Chemistry (0620) — 3.3 The mole and the Avogadro constant · Evaluate · 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 correct that both samples contain the same number of particles. The Avogadro constant (6.02×10²³) gives the number of particles per mole, so 2 mol of any substance contains 2×6.02×10²³ particles. However, the student is incorrect about the mass being the same. The mass of each sample depends on the relative atomic/molecular mass: 2 mol of H₂ has a mass of 4 g (2 mol×2 g mol⁻¹), whereas 2 mol of Ne has a mass of 40 g (2 mol×20 g mol⁻¹).

Examiner tips

  • State the particle equality first, then explain mass difference; use correct units and molar masses; keep answer concise to fit 4 marks

Common mistakes

  • Confusing number of particles with mass; using incorrect molar mass for H₂ or Ne; omitting the Avogadro constant in explanation

Mark scheme (4 marks)

  1. The student is correct that both samples contain the same number of particles
  2. The Avogadro constant (6.02 × 10²³) gives the number of particles per mole, so 2 mol of any substance contains 2 × 6.02 × 10²³ particles
  3. The student is incorrect about the mass being the same
  4. The mass of each sample depends on the relative atomic / molecular mass, so 2 mol of H₂ has a mass of 4 g whereas 2 mol of Ne has a mass of 40 g

Key terms in this question

mole

Related

More The mole and the Avogadro constant questions

▶ Try answering this question with AI marking (free) →