Explain how the relative formula mass of magnesium hydroxide, Mg(OH)₂, is determined from the relative atomic masses of its constituent elements, and state the relative formula mass of Mg(OH)₂. (Relative atomic masses: H = 1, O = 16, Mg = 24)

Cambridge International IGCSE Chemistry (0620) — 3.2 Relative masses of atoms and molecules · 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 relative formula mass of Mg(OH)₂ is found by adding the relative atomic masses of all the atoms in the formula.

Mg(OH)₂ contains 1 Mg, 2 O and 2 H atoms.

Relative formula mass = 24 + (2 × 16) + (2 × 1) = 24 + 32 + 2 = 58.

Thus the relative formula mass of Mg(OH)₂ is 58.

Examiner tips

  • Show the full calculation with the multiplication for each element.
  • Mention the number of each atom in the formula.
  • Use the correct units (amu) implicitly by giving the numerical value.
  • Keep the answer concise and to the point.

Common mistakes

  • Omitting the multiplication step and writing only 24 + 32 + 2 without explanation.
  • Using the wrong number of atoms (e.g., 1 O instead of 2).
  • Confusing relative atomic mass with atomic weight or using incorrect values.

Mark scheme (4 marks)

  1. Relative formula mass is found by adding together the relative atomic masses of all atoms in the formula
  2. Correct identification that there are one Mg, two O, and two H atoms in Mg(OH)₂
  3. Correct working: 24 + (2 × 16) + (2 × 1) OR 24 + 32 + 2
  4. Relative formula mass of Mg(OH)₂ = 58

Key terms in this question

relative formula mass · relative atomic mass

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