A technician uses paper chromatography to check the purity of a sample of a yellow food dye. The solvent front travels 9.0 cm. The yellow dye produces a single spot that travels 6.3 cm. A reference sample of pure yellow dye produces a single spot that travels 6.3 cm. Explain what the technician can conclude about the purity of the sample and describe how Rf values are used to identify the substances present.

Eduqas A-Level Chemistry — 3.7 Spectroscopy and chromatography · Explain · 5 marks · View as Markdown

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

Paper chromatography is used in quality control to check whether food dyes are pure. Each substance in a mixture travels a different distance up the chromatography paper, and its identity can be confirmed by calculating its Rf value.

Model answer (5 marks)

The technician observes only one spot on the chromatogram, so the sample contains a single substance and is therefore pure. The spot travels 6.3 cm while the solvent front travels 9.0 cm, giving an Rf value of 6.3÷9.0≈0.70. The reference sample of pure yellow dye gives the same Rf of 0.70, confirming that the unknown spot is the same substance as the reference. In paper chromatography each compound has a characteristic Rf value in a given solvent system; by comparing the Rf of an unknown with known reference values the substance can be identified.

Examiner tips

  • Show the calculation of Rf (distance moved ÷ solvent front) to get 0.70; match this to the reference value.
  • State that a single spot indicates purity and that matching Rf values confirm identity.
  • Mention that each compound has a unique Rf in a given solvent system.

Common mistakes

  • Calculating Rf incorrectly (e.g. using solvent front distance as numerator).
  • Failing to state that a single spot means the sample is pure.
  • Not comparing the sample Rf to the reference Rf to confirm identity.

Mark scheme (5 marks)

  1. The sample produces only one spot, so it contains only one substance / is a pure substance / is not a mixture
  2. The Rf value of the sample matches the Rf value of the reference / pure yellow dye, confirming the substance is the same
  3. Rf value = distance moved by substance ÷ distance moved by solvent front
  4. Correct Rf value calculated as 6.3 ÷ 9.0 = 0.70 (allow 0.7)
  5. Each substance has a characteristic / unique Rf value (in a given solvent), so Rf values can be compared to known reference values to identify unknowns

Key terms in this question

chromatography · Rf value · solvent front · reference

Related

More Spectroscopy and chromatography questions

▶ Try answering this question with AI marking (free) →