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 lab on spectroscopy gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Spectra are easier to predict when you know the functional group. In this lab you use quick test-tube reactions to identify three colourless liquids, then predict their mass, infrared and NMR spectra and compare each prediction with what the chemistry showed.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S3.2
Aim
To use three simple chemical tests to identify ethanol, propanone and ethanoic acid, then predict the mass, infrared and NMR spectra of each.
Equipment
- Three labelled liquids A, B and C (ethanol, propanone and ethanoic acid, 1 cm³ each)
- Six test tubes and a rack, three dropping pipettes
- 0.5 mol/dm³ sodium carbonate solution
- Acidified potassium dichromate(VI) solution, 0.1 mol/dm³ (teacher prepared)
- 2,4-dinitrophenylhydrazine solution (2,4-DNPH, teacher prepared)
- Hot water bath at 60 °C
- Gloves, eye protection
Safety
Wear gloves and eye protection throughout. Ethanol and propanone are highly flammable, so keep flames away. Dichromate(VI) is toxic and a suspected carcinogen, 2,4-DNPH is irritant and stains skin, and ethanoic acid is corrosive. Collect all waste in a labelled bottle.
Method
- Add 1 cm³ of each liquid to its own test tube labelled A, B or C.
- To a second set of three tubes, add 1 cm³ of each liquid again.
- To the first set, add 1 cm³ of sodium carbonate solution and note any bubbles.
- To the second set, add 5 drops of acidified potassium dichromate and warm in the water bath for 3 minutes.
- Place 2 drops of each liquid into fresh tubes and add 5 drops of 2,4-DNPH. Shake and observe.
- Record every observation in the table. Dispose of waste as instructed.
Results
| Liquid | Sodium carbonate | Warm acidified dichromate | 2,4-DNPH |
|---|---|---|---|
| A | no change | orange to green | no change |
| B | no change | no change, stays orange | orange precipitate |
| C | steady bubbles of gas | no change, stays orange | no change |
Analysis questions
Question 1 Identify the liquids
Use the results to identify A, B and C, and give the evidence for each.Question 2 Write the carbonate equation
Write a balanced equation for the reaction of ethanoic acid with sodium carbonate and name the gas.Question 3 Predict the IR spectra
State the key IR absorptions, in cm⁻¹, that would show A, B and C are the compounds you identified.Question 4 Predict the molecular ions
Calculate the m/z of the molecular ion for each liquid (C = 12, H = 1, O = 16).Question 5 Predict proton NMR
Give the number of proton NMR signals for each liquid and the splitting of the signals.Question 6 Predict 13C NMR
State the number of 13C signals for each liquid and say which carbon is at the highest shift.Question 7 Evaluate the method
Explain one limitation of the 2,4-DNPH test and suggest a way to improve identification.
