OxfordAQA A-Level Chemistry: Chromatography (9620) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

This set covers 3.3.16 Chromatography (subsection 3.3.16.1) of the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams from May/June 2020 onwards. Because this is International A2 content, all of these questions are International A-level only. Question 4 also uses 3.3.13.2 (locating amino acids), and questions 7 and 9 use precise masses from 3.3.6.2.

Learn the topic first from the study guide or the revision notes. The OxfordAQA A-Level Chemistry hub and the printable checklist cover the whole course, while the free diagnostics point to your weakest topics.

Data. Precise masses to use: C = 12.0000, H = 1.0078, N = 14.0031, O = 15.9949. Give Rf values to 2 decimal places.

Questions

1. Give two differences between thin-layer chromatography and gas chromatography in terms of the phases used. [2]

2. On a TLC plate, the solvent front is 6.4 cm above the baseline.

(a) Spot centres are at 1.2 cm and 3.7 cm. Work out both Rf values. [2] (b) In this same system, another compound has Rf = 0.38. How far above the baseline is the centre of its spot? [1]

3. A newly made compound gives a single spot on a TLC plate, so a student concludes that it is pure. Explain why this conclusion is not certain, and suggest how to obtain stronger evidence. [2]

4. A supplier’s sample of alanine is checked by TLC. It is spotted next to standards of alanine and glycine, and the plate is sprayed with ninhydrin after the run. The solvent front has risen 8.0 cm from the baseline.

Spot	Distance from baseline / cm
Glycine standard	1.6
Alanine standard	2.9
Sample, strong spot	2.9
Sample, faint spot	1.6

(a) Explain why the plate is sprayed with ninhydrin. [1] (b) Calculate the Rf value of each spot from the sample. [2] (c) State what the plate shows about the sample. Justify your answer. [2]

5. A mixture of ethyl benzoate, phenylmethanol (C₆H₅CH₂OH) and benzene-1,2-diol is separated by TLC. The plate’s coating is polar and the solvent has low polarity. Put the three compounds in order of Rf value, largest first, and explain the order. [3]

6. A cleaning fluid is analysed by gas chromatography. Standards run on the same column, at the same temperature and gas flow, give these retention times: propan-1-ol 3.4 min, butan-1-ol 5.7 min, pentan-1-ol 8.1 min. The cleaning fluid gives peaks at 3.4 min and 8.1 min only.

(a) State what is meant by retention time. [1] (b) Deduce which of the three alcohols the fluid contains. Explain your reasoning. [2] (c) The column temperature is raised for a second run, without rerunning the standards. Explain why the retention times above can no longer identify the peaks. [2]

7. A component separated by GC passes into a high-resolution mass spectrometer and gives a molecular ion at m/z = 84.0573. The possible formulae are C₆H₁₂, C₅H₈O, C₄H₈N₂ and C₄H₄O₂.

(a) Explain why measuring the m/z value only to the nearest whole number would not identify the formula. [1] (b) Show which formula is correct. [3]

8. A two-component mixture is run by TLC on a polar coating. The solvent front is 5.0 cm above the baseline. Two spots overlap, with centres at 4.7 cm and 4.8 cm.

(a) Calculate the Rf value of each spot. [1] (b) Explain, in terms of the two phases, why the separation is poor, and suggest a change of solvent that would improve it. [2]

9. A chemist makes pentyl ethanoate, CH₃COOCH₂CH₂CH₂CH₂CH₃, from ethanoic acid and pentan-1-ol. In GC, standards run under identical conditions give retention times of 2.2 min (ethanoic acid), 4.8 min (pentan-1-ol) and 7.4 min (pentyl ethanoate). The product gives peaks at 4.8 min, 7.4 min and 9.0 min.

(a) State what the peaks at 4.8 min and 7.4 min show about the product. [2] (b) The 9.0 min component is analysed by GC-MS. Explain why the mixture passes through the GC column before it reaches the mass spectrometer. [1] (c) The molecular ion of the 9.0 min component is at m/z = 130.0990. Use precise masses to decide between C₇H₁₄O₂, C₈H₁₈O and C₆H₁₄N₂O. [3] (d) Pentyl ethanoate is also C₇H₁₄O₂. Explain how the 9.0 min component can have a different retention time. [2]

10. Cyclohexanone and cyclohexane-1,2-diol are separated on a column packed with a polar solid. Six fractions are checked by TLC on one plate with standards of both. The solvent front is 7.0 cm above the baseline; the cyclohexanone standard is 3.8 cm and the diol standard 0.9 cm above it.

Fraction	TLC result
1	No spot
2 and 3	One spot, level with cyclohexanone
4	Two spots, level with both standards
5 and 6	One spot, level with cyclohexane-1,2-diol

(a) Calculate the Rf value of each standard. [2] (b) State which compound leaves the column first. Explain your answer. [2] (c) State which fractions should be combined to give each pure compound. [2]

Answers

1. Moving phase: a solvent in TLC but a gas in GC [1]. Stationary phase: a thin solid layer on a plate in TLC, but a packed column of solid, which may have a liquid coating, in GC [1]. [2] Examiner insight: Each difference must name both techniques; “GC uses a gas” alone says nothing about TLC.

2. (a) 1.2 / 6.4 = 0.19 [1]; 3.7 / 6.4 = 0.58 [1] (b) 0.38 × 6.4 = 2.4 cm [1] Examiner insight: In (b), the distance needs its unit; a bare 2.4 can lose the mark.

3. An impurity could have the same Rf value as the compound in this solvent, so the two would share one spot [1]. Rerun in different solvents (or use GC); one spot every time is stronger evidence [1]. [2] Examiner insight: “Repeat the experiment” is not a further test; it would give the same result.

4. (a) Amino acids are colourless; ninhydrin is a developing agent that makes their spots visible [1] (b) Strong spot: 2.9 / 8.0 = 0.36 [1]; faint spot: 1.6 / 8.0 = 0.20 [1] (c) The sample is mainly alanine but contains a small amount of an impurity, probably glycine [1]. The strong spot matches the alanine standard and the faint spot matches the glycine standard on the same plate [1] Examiner insight: In (c), “the sample is impure” without saying which spot matches which standard gains only the first mark.

5. Order: ethyl benzoate > phenylmethanol > benzene-1,2-diol [1]. Benzene-1,2-diol has two –OH groups, so it forms the most hydrogen bonds with the polar stationary phase and is retained most strongly [1]. Ethyl benzoate has no –OH group, so it is retained least and is the most soluble in the low-polarity solvent; phenylmethanol, with one –OH group, lies between [1]. [3] Examiner insight: An order with no explanation scores only the first mark; the reasons must compare the –OH groups.

6. (a) The time from injecting the sample into the column until the component reaches the detector [1] (b) Propan-1-ol and pentan-1-ol [1]. Their peaks match those standards under the same conditions; with no peak at 5.7 min, butan-1-ol is not detected [1] (c) Retention times depend on temperature: hotter, the components spend more time in the gas and come through sooner [1]. Peaks can only be matched to standards run under the same new conditions [1] Examiner insight: “The time taken to come out”, with no starting point, is usually too vague for (a).

7. (a) All four formulae have Mr = 84 to the nearest whole number [1] (b)

C6H12   : 6(12.0000) + 12(1.0078)             = 84.0936
C5H8O   : 5(12.0000) + 8(1.0078) + 15.9949    = 84.0573
C4H8N2  : 4(12.0000) + 8(1.0078) + 2(14.0031) = 84.0686
C4H4O2  : 4(12.0000) + 4(1.0078) + 2(15.9949) = 84.0210

Multiplying atom counts by precise masses and adding [1]; all four totals correct [1]; C₅H₈O is the only total equal to 84.0573: C₅H₈O [1] Examiner insight: A total for the matching formula alone is weak evidence; show every candidate’s total.

8. (a) 4.7 / 5.0 = 0.94 and 4.8 / 5.0 = 0.96 [1] (b) Both components are very soluble in this solvent and hardly retained, so both travel almost with the front [1]. Use a less polar solvent, so the polar stationary phase retains them more and their difference shows [1] Examiner insight: In (b), “use a different solvent” is too vague; the direction of the change carries the mark.

9. (a) The 4.8 min peak matches pentan-1-ol, so unreacted alcohol is present and the product is impure [1]. The 7.4 min peak matches pentyl ethanoate, so the ester has formed [1] (b) GC separates the components, so each enters the mass spectrometer on its own and gives its own spectrum [1] (c)

C7H14O2  : 7(12.0000) + 14(1.0078) + 2(15.9949)            = 130.0990
C8H18O   : 8(12.0000) + 18(1.0078) + 15.9949               = 130.1353
C6H14N2O : 6(12.0000) + 14(1.0078) + 2(14.0031) + 15.9949  = 130.1103

Correct method [1]; all totals correct [1]; formula C₇H₁₄O₂ [1] (d) It is a structural isomer of pentyl ethanoate [1]. Its different structure gives a different balance between the moving gas and retention by the stationary phase, so it reaches the detector at a different time [1] Examiner insight: In (d), the second mark needs the link to the balance between the phases.

10. (a) Cyclohexanone: 3.8 / 7.0 = 0.54 [1]; cyclohexane-1,2-diol: 0.9 / 7.0 = 0.13 [1] (b) Cyclohexanone [1]. The diol’s two –OH groups hydrogen bond to the polar solid, so it is retained more strongly; cyclohexanone’s balance favours the solvent (larger Rf), so it moves down faster [1] (c) Fractions 2 and 3 for cyclohexanone [1]; fractions 5 and 6 for cyclohexane-1,2-diol, leaving out fraction 4 because it contains both [1] Examiner insight: In (c), adding fraction 4 to either batch makes it impure and loses the second mark.

Where marks are usually lost

• Starting a measurement at the plate’s bottom edge, or ending it at the leading edge of a spot rather than its centre.
• Writing the Rf ratio upside down, or giving it units.
• Explaining separation with one phase only; credit needs both the solubility in the moving phase and the retention by the stationary phase.
• Describing a liquid as the moving phase in GC.
• Defining retention time without saying it starts at injection.
• Treating one spot as proof of purity, or one matching value as proof of identity.
• Showing the precise-mass total for only the matching formula.

Next steps

• Recap with the revision notes.
• Reread weak areas in the study guide.
• Extra precise-mass questions: organic analysis practice.
• See the OxfordAQA A-Level Chemistry hub and the printable checklist.
• Take a free 10-minute diagnostic.
• Book a free trial class.

Official syllabus

OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards (OxfordAQA). Section 3.3.16 Chromatography (International A2), subsection 3.3.16.1.