The method and the results are on the spectroscopy lab. Do the analysis yourself first, then check the answers below.
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. These answers go with the Chemistry 11-12 lab on spectroscopy: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
These answers use the sample observations. Your colours and timings may differ slightly, so base your conclusions on the pattern of positive and negative results, and check predicted spectra against the reference tables.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S3.2
A turns the dichromate from orange to green, so it is an oxidisable alcohol: ethanol. B gives an orange precipitate with 2,4-DNPH and no oxidation, so it is a ketone: propanone. C releases a gas with carbonate, so it is an acid: ethanoic acid.
Question 2 Write the carbonate equation
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. Atoms: C 4, H 8, O 6, Na 2 on both sides. The gas is carbon dioxide.
Question 3 Predict the IR spectra
Ethanol (A): broad O–H near 3300 cm⁻¹. Propanone (B): strong C=O near 1715 cm⁻¹ and no O–H. Ethanoic acid (C): very broad O–H from 2500 to 3300 cm⁻¹ plus strong C=O near 1710 cm⁻¹.
Ethanol: 3 signals (triplet 3H, quartet 2H, singlet 1H). Propanone: 1 signal, a singlet of 6H. Ethanoic acid: 2 signals, both singlets, 3H and 1H.
Question 6 Predict 13C NMR
Each liquid has 2 signals. The C=O carbon is highest: about 205 ppm in propanone and about 178 ppm in ethanoic acid. In ethanol the highest is the C–O carbon, near 58 ppm.
Question 7 Evaluate the method
2,4-DNPH reacts with both aldehydes and ketones, so it cannot tell them apart. A further test (Tollens’ reagent) or an IR/NMR spectrum is needed. Running a known sample alongside as a control also improves reliability.
