Quick answer: Mass spectrometry gives the relative molecular mass from the molecular ion and the structure from fragments, infrared spectroscopy identifies functional groups from bond absorptions, and proton and carbon-13 NMR show how many hydrogen and carbon environments exist and how they are joined. This Chemistry 11-12 test on spectroscopy has 8 questions worth 20 marks, to do in 45 minutes: reading a carbonyl band, halogen molecular ions and tertiary alcohol NMR. The mark scheme is on a separate page; free printable PDF.
This test covers mass spectra, infrared absorption, proton and carbon-13 NMR. Use the reference ranges in the lesson. Show all working for calculations, and write short, specific explanations.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S3.2
Question 1 Reading a carbonyl band (2 marks)
An IR spectrum has a strong absorption at 1735 cm⁻¹, an absorption at 2960 cm⁻¹ and nothing between 2500 and 3600 cm⁻¹.- Identify the bond responsible for 1735 cm⁻¹. (1 mark)
- Explain why the compound is not a carboxylic acid. (1 mark)
Question 2 Halogen molecular ions (3 marks)
The mass spectrum of 2-bromopropane, CH₃CHBrCH₃, has two molecular ion peaks.- Calculate their m/z values (C = 12, H = 1, ⁷⁹Br and ⁸¹Br). (2 marks)
- State the ratio of their heights and the reason. (1 mark)
Question 3 Tertiary alcohol NMR (3 marks)
2-methylpropan-2-ol is (CH₃)₃COH.- State the number of signals in its proton NMR spectrum and their integration ratio. (2 marks)
- Explain why both signals are singlets. (1 mark)
Question 4 Hexane isomers in 13C NMR (3 marks)
State the number of signals in the 13C NMR spectrum of each isomer of C₆H₁₄.- Hexane. (1 mark)
- 2,2-dimethylbutane. (1 mark)
- 2,3-dimethylbutane. (1 mark)
Question 5 Identify a small ester (3 marks)
A compound has M⁺ at m/z 74, formula C₃H₆O₂, and an IR absorption at 1745 cm⁻¹. Its proton NMR is shown.- Explain why there are two singlets of equal height. (2 marks)
- Name the compound. (1 mark)
Question 6 Wavenumber and bond strength (2 marks)
A C=O bond absorbs at 1715 cm⁻¹.- Calculate the wavelength in μm (1 μm = 10⁻⁴ cm). (1 mark)
- Explain why C=O absorbs at higher wavenumber than C=C. (1 mark)
Question 7 Deuterium oxide shake (2 marks)
A few drops of D₂O are shaken with ethanol and the proton NMR is run again. Predict what happens to the O–H signal and explain. (2 marks)Question 8 Counting carbons (2 marks)
In a mass spectrum, M⁺ has height 40.0 mm and the M+1 peak has height 3.08 mm. Each carbon contributes 1.1 %. Calculate the number of carbon atoms. (2 marks)
