Pentane and pentene are both organic compounds containing five carbon atoms. State the molecular formula of pentane and explain why pentane and pentene are not members of the same homologous series.

Cambridge International IGCSE Chemistry (0620) — 11.1 Formulae, functional groups and terminology · 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)

Pentane: C5H12
Pentane and pentene are not in the same homologous series because a homologous series has the same general formula and functional group. Pentane is an alkane (C–C single bonds, saturated) with the general formula CnH2n+2. Pentene is an alkene (contains a C=C double bond, unsaturated) with the general formula CnH2n. Their different general formulas and functional groups mean they belong to different series.

Examiner tips

  • State the exact formula C5H12 first. Explain the definition of a homologous series (same general formula/functional group). Show the difference in general formulae and bond type. Use the terms "alkane" and "alkene" to justify the separation.

Common mistakes

  • Writing the wrong formula (e.g. C5H10 for pentane). Forgetting to mention the double bond in pentene. Saying they are in the same series because they both have five carbons, ignoring the functional group difference.

Mark scheme (4 marks)

  1. Correct molecular formula of pentane: C₅H₁₂
  2. A homologous series has the same general formula / same functional group
  3. Pentane contains only C–C single bonds / is saturated, whereas pentene contains a C=C double bond / is unsaturated
  4. Pentane and pentene have different general formulae (CₙH₂ₙ₊₂ vs CₙH₂ₙ) so they belong to different homologous series (alkanes vs alkenes)

Key terms in this question

homologous series

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