Two successive members of a homologous series always differ by a –CH₂– unit. What is the corresponding difference in their molecular masses, and why is this value constant throughout the series?
Generated by claude-sonnet-4-6 · 2026-06-26 01:10 · grounding rag
Model Answer
Two successive members of a homologous series differ in molecular mass by 14 u.
This is because they differ by one –CH₂– unit, and the mass of C = 12 u and 2 × H = 2 u, giving 12 + 2 = 14 u.
This value is constant throughout the series because each successive member differs by exactly one –CH₂– unit, no more and no less.
Source: Chapter 4, Section 4.2.4 – Homologous Series
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Explanation
- The examiner expects you to state the value (14 u) and justify it using atomic masses (C = 12, H = 1 each, so CH₂ = 14).
- Always show the arithmetic: 12 + 2(1) = 14 u — this is where the mark lies.
- The second mark is for explaining why it is constant: every successive pair differs by exactly one –CH₂– unit, making the difference uniform across the series.
- Do not just say "14 u" without justification — that will cost you the reasoning mark.