A scientist uses mass spectrometry and infrared spectroscopy to analyse an unknown organic compound. Explain how these two techniques provide different types of information about the compound, and state what specific information each technique gives the scientist.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mass spectrometry can be used to find the relative formula mass of a compound. Infrared spectroscopy can be used to identify the functional groups present in a compound.
Model answer (5 marks)
Mass spectrometry gives the relative formula mass (Mr) of the compound. The molecular ion peak (M⁺) in the mass spectrum corresponds to the relative formula mass of the whole molecule, so the scientist can calculate Mr.
Infrared spectroscopy identifies the functional groups present in the compound. Different functional groups absorb infrared radiation at different (characteristic) frequencies/wavelengths, producing a spectrum that can be matched to known functional‑group bands.
Thus, mass spectrometry tells the scientist the size of the molecule (Mr) while infrared spectroscopy tells the scientist which functional groups are present, allowing the compound to be identified fully.
Infrared spectroscopy identifies the functional groups present in the compound. Different functional groups absorb infrared radiation at different (characteristic) frequencies/wavelengths, producing a spectrum that can be matched to known functional‑group bands.
Thus, mass spectrometry tells the scientist the size of the molecule (Mr) while infrared spectroscopy tells the scientist which functional groups are present, allowing the compound to be identified fully.
Examiner tips
- Use the command word ‘Explain’ – describe what each technique does and the type of data it gives.
- Show the key point that MS gives Mr via the M⁺ peak; IR gives functional‑group information via characteristic bands.
- Mention that together they provide both size and structure.
- Keep the answer concise – 5 marks, so one sentence per point.
Common mistakes
- Confusing the M⁺ peak with fragment peaks – only M⁺ gives Mr.
- Saying IR gives the whole structure – it only identifies functional groups.
- Using vague terms like ‘molecular weight’ instead of ‘relative formula mass’.
Mark scheme (5 marks)
- Mass spectrometry gives the relative formula mass (Mr) of the compound
- The molecular ion peak (M+) in the mass spectrum corresponds to the relative formula mass of the whole molecule
- Infrared spectroscopy identifies the functional groups present in the compound
- Different functional groups absorb infrared radiation at different (characteristic) frequencies/wavelengths
- Together the two techniques allow the scientist to determine both the size of the molecule (Mr) and its functional groups, helping to identify the compound fully
Key terms in this question
mass spectrometry · infrared spectroscopy
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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