Mass spectrometry for structure determinationEdexcel A-Level Chemistry: Subtopic test

1. 1
   Carbon monoxide (CO), nitrogen (N₂) and ethene (C₂H₄) all have a nominal relative molecular mass of 28. An environmental chemist uses a high-resolution mass spectrometer to analyse a sample of exhaust gas. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.

   (a)

   Which value is the accurate relative molecular mass of carbon monoxide, ¹²C¹⁶O, to four decimal places?

   [1 mark]

   • A28.0000
   • B28.0062
   • C27.9949
   • D28.0312

   (b)

   Why do CO, N₂ and C₂H₄ have different accurate relative molecular masses even though their nominal masses are the same?

   [1 mark]

   • AIsotopic masses are not exact whole numbers, so different combinations of atoms give different totals
   • BElectrons are lost in ionisation, and the number lost differs between the molecules
   • CEach molecule contains a different isotope of carbon
   • DThe spectrometer is calibrated differently for each gas

   (c)

   A second sample gives a molecular ion peak at m/z 28.0312. Show by calculation which of the three gases it contains.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A compound gives a molecular ion peak at m/z 58.0417 in a high-resolution mass spectrum. Four possible molecular formulae with a nominal relative molecular mass of 58 are C₂H₂O₂, C₂H₆N₂, C₃H₆O and C₄H₁₀. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.

   (a)

   What is the accurate relative molecular mass of C₄H₁₀ to four decimal places?

   [1 mark]

   • A58.0417
   • B58.0530
   • C58.0054
   • D58.0780

   (b)

   Why can the chemist reject C₄H₁₀ as the molecular formula of the compound?

   [1 mark]

   • AIts nominal relative molecular mass is not 58
   • BIts accurate Mᵣ, 58.0780, does not match the measured value, 58.0417
   • CIt contains no oxygen, so it cannot give a molecular ion peak
   • DHydrocarbons cannot be detected by mass spectrometry

   (c)

   Show by calculation that the accurate relative molecular mass of C₃H₆O matches the measured peak, and state why a low-resolution mass spectrum could not distinguish C₃H₆O from C₄H₁₀.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A gas sample is known to be ethanol vapour (C₂H₆O), methanoic acid vapour (CH₂O₂) or nitrogen dioxide (NO₂). All three have a nominal relative molecular mass of 46. A high-resolution mass spectrum of the sample shows a molecular ion peak at m/z 46.0054. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.

   (a)

   Calculate the accurate relative molecular mass of each of the three possible compounds to four decimal places, and use your values to identify the compound in the sample.

   [3 marks]

   (b)

   Explain why ethanol and methanoic acid have different accurate relative molecular masses although their nominal masses are the same, and why a high-resolution instrument is needed to distinguish them.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pharmaceutical analyst isolates a pure white solid from a tablet. The solid contains only carbon, hydrogen, nitrogen and oxygen. High-resolution mass spectrometry gives a molecular ion peak at m/z 151.0631. Three formulae with nominal relative molecular mass 151 are possible: C₈H₉NO₂, C₇H₉N₃O and C₉H₁₃NO. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.

   (a)

   Calculate the accurate relative molecular mass of each of the three possible formulae to four decimal places, decide which formula the compound has, and explain why a low-resolution mass spectrum could not make this decision.

   [6 marks]

   (b)

   Paracetamol, HOC₆H₄NHCOCH₃, and methyl 2-aminobenzoate, H₂NC₆H₄COOCH₃, are both isomers of C₈H₉NO₂. Evaluate how far an accurate relative molecular mass alone can identify the compound in the tablet.

   [6 marks]

   Total for question 4: 12 marks

End of questions