A Level Chemistry: Chromatography — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Chromatography revision notes, which covers TLC and GLC in full, plus GC-MS as enrichment beyond the specification.

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Questions

1. Explain the general principle by which any form of chromatography separates a mixture. [3]

2. In thin-layer chromatography, a spot moves 4.6 cm while the solvent front moves 7.8 cm.

(a) Calculate the R_f value. [2] (b) Explain why an R_f value is only meaningful when the conditions are stated. [2] (c) Two different compounds give the same R_f. Explain why this does not prove they are identical, and state what should be done. [2]

3. Explain each practical instruction, giving the reason:

(a) the baseline is drawn in pencil [2] (b) the spot must start above the solvent level [2] (c) the tank is covered with a lid [2]

4. In gas–liquid chromatography:

(a) Name the mobile and stationary phases. [2] (b) State two factors that determine retention time. [2] (c) Explain what the peak area represents. [2] (d) State one limitation of the technique. [1]

5. A GLC trace of a three-component mixture gives peak areas of 12.0, 28.0 and 20.0 (arbitrary units) for P, Q and R respectively. Calculate the percentage composition by peak area of component Q. [2]

6. (Background — visualisation methods are not required recall for 9701; included for practical context only, no marks awarded.) A student uses TLC on colourless amino acids. Suggest how the spots could be made visible.

7. A silica TLC plate is used to separate two components: one polar, one non-polar. Explain which component travels further up the plate, and why. [3]

8. In gas-liquid chromatography, explain why a component with a low boiling point and low solubility in the stationary phase has a shorter retention time than one with a high boiling point and high solubility. [3]

9. Two spots on a TLC plate have R_f values of 0.32 and 0.68, using the same solvent system, in which the solvent front travelled 9.0 cm. Calculate the distance travelled by each spot. [2]

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Answers

1. The components have different affinities for the two phases [1]. A component with a greater affinity for the stationary phase moves more slowly [1]; one with a greater affinity for the mobile phase moves further, so components with different affinities end up spatially separated [1].

2. (a) R_f = 4.6 ÷ 7.8 [1] = 0.59 [1]. (b) R_f depends on the solvent, stationary phase and temperature [1], so a value obtained under different conditions cannot be compared [1]. (c) Two different compounds can coincidentally have the same R_f [1]; a known reference sample should be run alongside on the same plate, or a second solvent system used [1].

3. (a) Ink would dissolve in the solvent [1] and travel up the plate, contaminating the chromatogram [1]. (b) Otherwise the spot would dissolve into the solvent reservoir [1] rather than being carried up the plate [1]. (c) To saturate the tank with solvent vapour [1], preventing evaporation from the plate which would alter the solvent front and the R_f values [1].

4. (a) Mobile: an inert carrier gas such as nitrogen or helium [1]. Stationary: a high-boiling liquid on an inert solid support [1]. (b) The component’s boiling point [1] and its solubility in the stationary phase [1]. (c) The amount (or concentration) of that component in the mixture [1], so the technique is quantitative once calibrated against known standards [1]. (d) The sample must be volatile and thermally stable [1].

5. Total area = 12.0 + 28.0 + 20.0 = 60.0 [1]; percentage of Q = (28.0 ÷ 60.0) × 100% = 46.7% [1].

6. (Background, not assessed.) A locating agent such as ninhydrin reacts with amino acids to give a characteristic purple/blue colour, or the plate can be viewed under ultraviolet light if it contains a fluorescent indicator.

7. The non-polar component travels further, giving it the higher R_f value [1]. Silica is a polar stationary phase [1], so the polar component is retained more strongly on it, while the non-polar component has a greater affinity for the mobile phase (solvent) and is carried further up the plate [1].

8. A component with a low boiling point vaporises more readily and spends less time condensed in the stationary phase [1]; low solubility in the stationary phase means it spends less time dissolved there too [1], so it is carried through the column by the mobile gas phase faster, giving a shorter retention time [1].

9. distance = R_f × solvent-front distance: 0.32 × 9.0 = 2.9 cm [1]; 0.68 × 9.0 = 6.1 cm [1].

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Where marks are usually lost

• Comparing R_f values from different conditions.
• Giving practical instructions without the reason.
• Saying a matching R_f proves identity.
• Forgetting that GLC requires a volatile, thermally stable sample.
• Explaining Rf differences without linking polarity to which phase the component is attracted to — “it moves further” is not a reason on its own.
• Assuming retention time depends only on boiling point — solubility in the stationary phase matters just as much.
• (Background point, not examinable) Ninhydrin only develops colourless amino-acid spots; it is not needed for spots that are already coloured.

Work through these alongside the chromatography revision notes: the notes set out the principle and the definitions, while these questions test whether you can apply the polarity and volatility arguments to a specific pair of compounds rather than just recite the theory.