Mass spectrometry for structure determinationEdexcel A-Level Chemistry: Mind map

Exam questions on Mass spectrometry for structure determination

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 second sample gives a molecular ion peak at m/z 28.0312. Show by calculation which of the three gases it contains.2 marks
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.
   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
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.
   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