Mass spectrometry for structure determinationEdexcel A-Level Chemistry: Flashcards
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What does the molecular ion peak show?
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- What does the molecular ion peak show?
- Its m/z equals the relative molecular mass of the compound.
- What is the molecular ion?
- The ion M⁺ formed when a molecule loses one electron.
- What is the difference between nominal and accurate mass?
- Nominal is to the nearest whole number; accurate is given to four decimal places.
- Why are accurate masses not whole numbers?
- Isotope masses are not whole numbers on the ¹²C = 12.0000 scale.
- Give the accurate masses of ¹H, ¹⁴N and ¹⁶O.
- 1.0078, 14.0031 and 15.9949.
- What is the accurate Mᵣ of CO?
- 12.0000 + 15.9949 = 27.9949.
- What is the accurate Mᵣ of N₂?
- 2 × 14.0031 = 28.0062.
- What is the accurate Mᵣ of C₂H₄?
- 2 × 12.0000 + 4 × 1.0078 = 28.0312.
- What is the accurate Mᵣ of C₃H₆O?
- 36.0000 + 6.0468 + 15.9949 = 58.0417.
- How do you identify a compound from an accurate molecular ion peak?
- Calculate the accurate Mᵣ of each candidate formula and pick the one that matches.
- Why can a low-resolution spectrum not distinguish C₃H₆O from C₄H₁₀?
- Both have nominal Mᵣ 58, so both give a peak at m/z 58.
- Why can accurate Mᵣ not distinguish isomers?
- Isomers have the same molecular formula and therefore the same accurate Mᵣ.
- What does accurate Mᵣ establish about an unknown?
- Its molecular formula, which is then combined with other data to find the structure.
Exam questions on Mass spectrometry for structure determination
- 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
- 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
- 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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).