A chemist is investigating an unknown ionic compound. The compound contains only magnesium and oxygen. The chemist determines that the compound contains 60.3% magnesium and 39.7% oxygen by mass. Use this information to determine the empirical formula of the compound, showing your reasoning clearly.

OCR A-Level Chemistry A (H432) — 2.2 Amount of substance · Justify · 5 marks · View as Markdown

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

Magnesium has a relative atomic mass of 24.3. Oxygen has a relative atomic mass of 16.0.

Model answer (5 marks)

1. 60.3 % Mg ÷ 24.3 g mol⁻¹ = 2.48 mol Mg
2. 39.7 % O ÷ 16.0 g mol⁻¹ = 2.48 mol O
3. 2.48 ÷ 2.48 = 1 : 1 (simplest ratio)
4. Empirical formula = MgO
5. An empirical formula is the simplest whole‑number ratio of atoms of each element in a compound.

Examiner tips

  • Show the two division steps and the ratio calculation clearly; use the word ‘simplest’ when stating the formula.
  • Explain that the empirical formula is the whole‑number ratio of atoms, not the molar mass.

Mark scheme (5 marks)

  1. Divides percentage of magnesium by its relative atomic mass: 60.3 ÷ 24.3
  2. Divides percentage of oxygen by its relative atomic mass: 39.7 ÷ 16.0
  3. Divides both values by the smallest to find the simplest ratio
  4. States the correct empirical formula as MgO
  5. Correctly defines empirical formula as the simplest whole number ratio of atoms of each element in the compound

Key terms in this question

empirical formula

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