Mass spectrometry in organic analysisEdexcel A-Level Chemistry: Subtopic test

1. 1
   A chemist analyses a liquid that contains carbon, hydrogen and oxygen only. In its mass spectrum the peak with the highest m/z value is at 58, and there are strong fragment peaks at m/z 43 and m/z 15. There are no strong peaks at m/z 29 or 57.

   (a)

   What species is responsible for the peak at m/z 58?

   [1 mark]

   • AThe molecular ion, formed when a molecule loses one electron
   • BA fragment ion formed by loss of a CH₃ group
   • CA molecule that has gained one electron
   • DThe base peak, which is always the molecular ion

   (b)

   Which ion is responsible for the peak at m/z 43?

   [1 mark]

   • A[CH₃]⁺
   • B[C₂H₅]⁺
   • C[CH₃CO]⁺
   • D[CH₃COCH₃]⁺

   (c)

   Deduce the identity of the compound, explaining how the peaks at m/z 58, 43 and 15 support your answer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Pentan-2-one and pentan-3-one are structural isomers with molecular formula C₅H₁₀O. A student uses mass spectrometry to tell them apart.

   (a)

   What is the m/z value of the molecular ion peak of both isomers?

   [1 mark]

   • A70
   • B86
   • C84
   • D87

   (b)

   Which pair of m/z values corresponds to the two acylium ions, R–C≡O⁺, formed by cleavage on either side of the C=O group in pentan-2-one?

   [1 mark]

   • A29 and 57
   • B15 and 85
   • C57 and 71
   • D43 and 71

   (c)

   Explain how the mass spectra would allow the student to distinguish between the two isomers.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is given an unknown liquid X. Elemental analysis shows that it contains 64.9% carbon, 13.5% hydrogen and 21.6% oxygen by mass. In its mass spectrum the peak with the highest m/z value is at 74 and there is a strong fragment peak at 59. X is not oxidised by warm acidified potassium dichromate(VI). X reacts with sodium metal to give hydrogen gas.

   (a)

   Determine the empirical formula and the molecular formula of X.

   [3 marks]

   (b)

   Deduce the structure of X. Use the fragment at m/z 59 and the oxidation result in your answer, and give the formula of the ion at m/z 59.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic laboratory uses mass spectrometry to identify organic compounds recovered from a crime scene. Only positive ions are detected in each spectrum.

   (a)

   Compound Z contains 54.5% carbon and 9.1% hydrogen by mass, the remainder being oxygen. Its mass spectrum has the peak of highest m/z at 88 and fragment peaks at m/z 45 and m/z 43. Z does not react with sodium metal. Deduce the molecular formula of Z and suggest a structure for it, explaining how each piece of data supports your answer.

   [6 marks]

   (b)

   A student states: "Propan-1-ol and propan-2-ol both have a molecular ion peak at m/z 60, so mass spectrometry cannot tell them apart." Evaluate this statement, giving the m/z values and ions you would expect for each alcohol.

   [6 marks]

   Total for question 4: 12 marks

End of questions