CCEA A Level Chemistry
CCEA A Level Chemistry 5.1 Mass Spectrometry Exam Questions
Free CCEA A Level Chemistry revision for 5.1 Mass Spectrometry: what you need to know, common mistakes and exam-style questions with answers. Covers molecular ion and base peaks, the M+1 peak, fragment ions, suggesting fragment formulae and high resolution mass spectrometry.
Practise 5.1 Mass Spectrometry with past papers
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About CCEA A Level Chemistry 5.1 Mass Spectrometry
Section 5.1 opens Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nitrogen Chemistry. It extends the mass spectra of elements from AS 1 (1.2) to organic molecules. You need to identify the molecular ion peak, the base peak, the M+1 peak and fragment ions, suggest formulae for fragments from their m/z values, and explain how high resolution mass spectrometry distinguishes molecules with the same integer relative molecular mass. Mass spectrometry is usually combined with infrared (2.7) and NMR (5.2) in structure determination questions, where you must use all the evidence together. Accurate formulae of ions, including the positive charge, are essential.
What you need to know
- Molecular ion The peak with the highest m/z (ignoring the small M+1 peak) is the molecular ion, M+, and gives the Mr of the compound.
- Base peak The tallest peak in the spectrum, given a relative abundance of 100. It corresponds to the most stable or most abundant ion, which is often a fragment.
- M+1 peak A small peak one unit above M+, caused by molecules containing a carbon-13 atom. Its height relative to M+ increases with the number of carbon atoms (about 1.1% per carbon).
- Fragmentation The molecular ion breaks into a positive ion and a neutral radical; only the positive ion is detected. Common fragments: CH3+ at 15, C2H5+ at 29, CH3CO+ at 43, and loss of 15 (CH3) or 29 (C2H5) from M.
- High resolution Measuring m/z to four or more decimal places distinguishes molecules with the same integer Mr, because exact atomic masses differ (for example CO 27.9949, N2 28.0061, C2H4 28.0313).
Common mistakes in 5.1 Mass Spectrometry
- Writing fragment ions without a positive charge. Every detected species is a positive ion, for example CH3CO+.
- Taking the base peak as the molecular ion. The base peak is the most abundant ion; the molecular ion is at the highest m/z.
- Saying the M+1 peak comes from hydrogen-2. It is mainly due to carbon-13.
- Showing both fragments as ions. One fragment is a positive ion and the other an uncharged radical.
- Using integer masses when the question gives exact masses. Use the precise values to choose between formulae.
Try three 5.1 Mass Spectrometry exam-style questions
1. The mass spectrum of a ketone has a molecular ion at m/z 58 and a base peak at 43. Identify the ketone and the ion at 43. (2 marks)
Show the mark scheme answer
Propanone, CH3COCH3; CH3CO+.
2. A compound has a molecular ion at m/z 44.0262. Which of CO2 (43.9898), C3H8 (44.0626) and C2H4O (44.0262) is it? (1 mark)
Show the mark scheme answer
C2H4O.
3. Write an equation for the fragmentation of the molecular ion of ethanol to give a peak at m/z 31. (2 marks)
Show the mark scheme answer
[CH3CH2OH]+ gives CH2OH+ + CH3. ; correct charges and radical shown.
5.1 Mass Spectrometry FAQs
What does CCEA A Level Chemistry 5.1 Mass Spectrometry cover?
It covers molecular ion, base and M+1 peaks, fragment ions and high resolution mass spectrometry for organic molecules.
Which unit is 5.1 Mass Spectrometry examined in?
It is examined in Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nitrogen Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
How do I use mass spectra in structure questions?
Find Mr from the molecular ion, then use the main fragments (losses of 15, 29 or 43, and peaks such as 43 for CH3CO+) together with IR and NMR evidence to build the structure.
