Glucose has the molecular formula C₆H₁₂O₆. State the empirical formula of glucose and explain why the empirical formula of glucose is different from its molecular formula.

Cambridge International IGCSE Chemistry (0620) — 3.1 Formulae · State 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)

The empirical formula of glucose is CH₂O.

The empirical formula gives the simplest whole‑number ratio of the atoms of each element in the compound.

The molecular formula shows the actual number of atoms of each element in one molecule of the substance.

The ratios of C : H : O in C₆H₁₂O₆ can be divided by 6, giving the simpler formula CH₂O, which is therefore different from the molecular formula.

Examiner tips

  • State the empirical formula first; then give three distinct points explaining the difference.
  • Use the words ‘simplest ratio’ and ‘actual number of atoms’ – these are key phrases in the mark scheme.
  • Show the division by 6 to illustrate the simplification.
  • Keep each point concise and separate them with line breaks or numbering.

Common mistakes

  • Writing the empirical formula as C₃H₆O₃ instead of CH₂O.
  • Failing to explain that the empirical formula is a ratio, not a count of atoms.
  • Using vague terms like ‘smaller’ or ‘different’ without specifying the reason for the difference.

Mark scheme (4 marks)

  1. Empirical formula of glucose is CH₂O
  2. The empirical formula shows the simplest whole number ratio of atoms of each element in the compound
  3. The molecular formula shows the actual number of atoms of each element in one molecule
  4. The ratios of C : H : O in C₆H₁₂O₆ can be simplified / divided by 6, giving a different (simpler) formula to the molecular formula

Key terms in this question

empirical formula · molecular formula

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