OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 9

1 mark · Easy difficulty · Multiple Choice

Determine the molecular formula of a given branched cycloalkane structure from multiple-choice options.

Practise this question

Question

Multiple choice question 9 asks for the molecular formula of the compound shown as a skeletal structure representing 1,2-dimethylcyclopentane. Below are four options: A (C7H10), B (C7H12), C (C7H14), and D (C7H16), along with a box for the answer.
Question text

9 What is the molecular formula of the compound below?

A C7H10

B C7H12

C C7H14

D C7H16

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme table indicates the correct answer for question 9 is option C, worth 1 mark.

9 C 1

How to answer it

Determining Molecular Formula from Skeletal Structures

What this question tests

This question tests your ability to interpret skeletal formulae of cyclic organic molecules, count carbon and hydrogen atoms accurately, and apply the general formula of cycloalkanes to find the correct molecular formula.

Question 9

Part 9: Finding the Molecular Formula

✅ Correct Answer

The correct option is C ( C₇H₁₄ ).

Mark Awarded: 1 / 1

💡 Key Knowledge

  • Skeletal Formula: Vertices and line endpoints represent carbon atoms. Hydrogen atoms attached to carbons are implied and not drawn.
  • Ring Systems: A single ring introduces one ring of unsaturation, meaning a monocyclic alkane has the general formula CₙH₂ₙ (the same as alkenes).

🧠 Exam Technique

  • Count Carbons First: Count every corner of the pentagon (5 carbons) plus each protruding single line for the methyl groups (2 carbons). Total = 7 carbons ( C₇ ).
  • Use General Formulae: Since the compound is a monocyclic alkane (cyclopentane ring with alkyl substituents), it has one ring and no double bonds. Therefore, apply CₙH₂ₙ directly: if n = 7 , hydrogens = 7 × 2 = 14 .

❌ Common Errors

  • Forgetting to count the hydrogens on the substituent methyl groups correctly.
  • Confusing cycloalkanes with straight-chain alkanes ( CₙH₂ₙ₊₂ ) and selecting option D ( C₇H₁₆ ). Rings reduce the hydrogen count by 2, exactly like a carbon-carbon double bond does.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.