OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 20
1 mark · Medium difficulty · Multiple Choice
Identify the carboxylic acid that produces a mass spectrum with peaks at m/z = 29 and m/z = 102 from four given structural isomers.
Practise this questionQuestion
Question text
20 The mass spectrum of a carboxylic acid contains peaks at m/z = 29 and m/z = 102.
Which compound could have produced the spectrum?
O
A
OH
OH
B
O
OH
C
O
OH
D
O
Your answer [1]
Mark scheme
Show the mark scheme
20 C 1 2.6
Total 20
SECTION B
How to answer it
Mass Spectrometry & Molecular Ion Identification
This question assesses your understanding of mass spectrometry, specifically the identification of molecular ion peaks (M⁺) to find relative molecular mass (Mᵣ), and the recognition of common fragment ions (such as m/z = 29 corresponding to an ethyl or formyl fragment) to deduce the structural isomer of a carboxylic acid.
Question 20 Analysis
Identifying the Correct Carboxylic Acid Isomer
✅ Correct Answer: C
Compound C is 2-methylbutanoic acid (C₅H₁₀O₂). It has a molecular mass of (5 × 12.0) + (10 × 1.0) + (2 × 16.0) = 102 , matching the molecular ion peak at m/z = 102. It also contains an ethyl branching group that readily cleaves to form a stable fragment ion with m/z = 29 (C₂H₅⁺).
💡 Key Knowledge
- Molecular Ion Peak (M⁺): The peak with the highest m/z value corresponds to the Mr of the intact molecule. Here, Mr = 102.
- Isomer Mass Check: Isomers A, B, C, and D all share the molecular formula C₅H₁₀O₂ and thus all have an Mr of 102. You must use fragmentation patterns to distinguish them.
- Common Fragments: m/z = 29 typically represents either an C₂H₅⁺ group (ethyl cation) or an CHO⁺ group.
🧠 Exam Technique
When multiple options share the exact same molecular formula and Mr, scan the structures for alkyl branches or functional groups that can easily fragment to produce standard carbocations (e.g., m/z 15 for CH₃⁺, m/z 29 for C₂H₅⁺, m/z 43 for C₃H₇⁺).
❌ Common Errors
- Stopping at the molecular mass calculation and guessing blindly when all options share Mr = 102.
- Misidentifying which carbon-carbon bonds undergo alpha-cleavage in branched carboxylic acids.
Topics
Module 6: Organic chemistry and analysis · 6.3 Analysis
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.