A scientist is testing two painkiller tablets from different manufacturers. Both tablets claim to contain the same active ingredient. The scientist uses paper chromatography with the same solvent to test samples from each tablet alongside a reference sample of the pure active ingredient. The pure active ingredient produces a single spot. Tablet A produces a single spot at the same position as the reference. Tablet B produces two spots, one of which is at the same position as the reference. Explain what the chromatography results tell the scientist about each tablet, and describe how the Rf value could be used to help confirm the identity of the active ingredient.

WJEC A-Level Chemistry (Wales) — 1.2 Electronic structure · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Tablet A contains only one substance – it is a pure compound. The single spot appears at the same Rf position as the reference, proving it contains the active ingredient.
Tablet B is a mixture, as two distinct spots are observed. One spot matches the reference, so the active ingredient is present; the other spot represents an additional compound.
The Rf value is calculated as the distance travelled by the substance divided by the distance travelled by the solvent front. Each compound has a characteristic Rf value in a given solvent; comparing the measured Rf to the known reference value confirms the identity of the active ingredient.

Examiner tips

  • Use the command word ‘explain’ – give reasons and link to the data; mention purity, mixture and identity; state how Rf is calculated and used for confirmation.
  • Show the Rf calculation formula and note that the same solvent system gives comparable values.
  • Include the fact that the reference spot matches the tablet A spot to prove identity.
  • Mention that the second spot in tablet B indicates an impurity or excipient.

Common mistakes

  • Confusing Rf with retention factor (k) or forgetting to divide by solvent front distance.
  • Saying tablet B is pure because it contains the active ingredient, ignoring the extra spot.
  • Not explaining that the Rf value is characteristic and used for comparison to a reference value.

Mark scheme (5 marks)

  1. Tablet A contains only one substance / is a pure substance
  2. Tablet A's spot is at the same position as the reference, so it contains the active ingredient
  3. Tablet B is a mixture / contains more than one substance
  4. Tablet B contains the active ingredient AND another substance / an additional compound
  5. The Rf value is calculated as distance travelled by the substance divided by the distance travelled by the solvent, and each compound has a characteristic Rf value in a given solvent which can be compared to known reference values to confirm identity

Key terms in this question

paper chromatography · Rf value

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