Methane, ethane and propane are the first three members of the alkane homologous series. State the molecular formula of propane and explain what is meant by the term 'homologous series', referring to functional groups and chemical properties in your answer.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.1 Introduction · 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)

Propane has the molecular formula C₃H₈.
A homologous series is a group of compounds that all contain the same functional group – in this case the alkane – and therefore have the same general formula CₙH₂ₙ₊₂. Members of a homologous series show similar chemical properties because they share that functional group. Each successive member of the series differs from the previous one by a CH₂ unit (one carbon and two hydrogen atoms).

Examiner tips

  • State the formula first, then give the definition of a homologous series, mention the common functional group, note similar properties, and finish with the CH₂ difference.

Common mistakes

  • Writing the wrong formula (e.g. C₃H₆), forgetting to mention the functional group, or not stating the CH₂ difference between members.

Mark scheme (4 marks)

  1. Correct molecular formula of propane: C₃H₈
  2. A homologous series is a series of compounds with the same functional group
  3. Members of a homologous series have similar chemical properties
  4. Each successive member differs by CH₂ (one carbon and two hydrogen atoms)

Key terms in this question

molecular formula · homologous series · functional group

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