Mass spectra and infrared spectroscopyEdexcel International A Level Chemistry: Subtopic test

1. 1
   A compound X contains carbon, hydrogen and oxygen only. Its mass spectrum has a molecular ion peak at m/z 58, the largest fragment peak at m/z 43, a small peak at m/z 15 and no significant peak at m/z 29. Its infrared spectrum has a strong absorption at 1715 cm⁻¹ and no absorption above 3000 cm⁻¹.

   (a)

   Which statement about the peak at m/z 58 is correct?

   [1 mark]

   • AIt is the most abundant ion, so it is the base peak
   • BIt is the molecular ion peak, so the relative molecular mass of X is 58
   • CIt is formed when X loses a CH₃ group
   • DIt shows that X contains 58 carbon atoms

   (b)

   Which compound is X?

   [1 mark]

   • APropanone, CH₃COCH₃
   • BPropanal, CH₃CH₂CHO
   • CButane, CH₃CH₂CH₂CH₃
   • DPropan-1-ol, CH₃CH₂CH₂OH

   (c)

   Give the formulae of the ions responsible for the peaks at m/z 43 and m/z 15.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A liquid Z has an infrared spectrum with a very broad absorption between 2500 and 3300 cm⁻¹ and a strong, sharp absorption at 1710 cm⁻¹. Its molecular ion peak is at m/z 74.

   (a)

   Which compound could Z be?

   [1 mark]

   • APropan-1-ol
   • BPropanal
   • CPropanone
   • DPropanoic acid

   (b)

   Which bond causes the strong, sharp absorption at 1710 cm⁻¹?

   [1 mark]

   • AO–H
   • BC–H
   • CC=O
   • DC=C

   (c)

   A student suggests that Z is an alcohol. Use the infrared data to explain why this is not correct.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A compound W contains only carbon, hydrogen and oxygen. Its mass spectrum has peaks at m/z 74, 57, 45 and 29, with the peak at m/z 74 the molecular ion. Its infrared spectrum has a very broad absorption at 2500 to 3300 cm⁻¹ and a strong absorption at 1710 cm⁻¹.

   (a)

   Use the infrared data to deduce the functional group in W and state the evidence for it.

   [3 marks]

   (b)

   Deduce the structure of W and identify the ions responsible for the peaks at m/z 57, 45 and 29.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic chemist uses infrared (IR) spectroscopy and mass spectrometry to identify organic liquids. Typical IR absorptions: O–H in alcohols 3200–3550 cm⁻¹ (broad); O–H in carboxylic acids 2500–3300 cm⁻¹ (very broad); C=O 1680–1750 cm⁻¹; C=C 1620–1680 cm⁻¹; C–H in aldehydes 2700–2900 cm⁻¹; C–H in alkanes and alkenes 2850–3100 cm⁻¹.

   (a)

   An unknown liquid X has a molecular ion at m/z 60, its largest fragment peak at m/z 45 and no peak at m/z 31. Its IR spectrum has a broad absorption at 3200–3550 cm⁻¹ and no absorption between 1680 and 1750 cm⁻¹. Deduce the identity of X, explaining the evidence from each spectrum.

   [6 marks]

   (b)

   Propan-1-ol is oxidised first to propanal and then to propanoic acid. Describe how IR spectroscopy and the molecular ion peaks could be used to show which of these three compounds a sample contains.

   [6 marks]

   Total for question 4: 12 marks

End of questions