A student carries out paper chromatography on two samples, sample A and sample B, using the same solvent. Sample A produces a single spot that travels 4.8 cm from the baseline. The solvent front travels 6.0 cm from the baseline. Sample B produces three separate spots that travel 1.5 cm, 3.0 cm and 4.8 cm from the baseline. Explain what these results tell the student about the nature of sample A and sample B, and calculate the Rf value of the spot in sample A, stating what this value could be used for.

Eduqas A-Level Chemistry — 1.7 Equilibria · Explain · 5 marks · View as Markdown

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

Paper chromatography is used to separate and identify substances in a mixture. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.

Model answer (5 marks)

Sample A gives a single spot, so it is a pure substance. Sample B gives three spots, so it is a mixture of at least three components. The Rf value of the spot in sample A is 4.8 cm ÷ 6.0 cm = 0.80. This Rf value can be used to identify the substance by comparing it with literature or a reference sample.

The different Rf values in sample B show that the components have different solubilities in the solvent, which causes them to travel different distances up the paper.

Examiner tips

  • State that a single spot indicates purity, multiple spots indicate a mixture
  • Show the Rf calculation with units
  • Explain that Rf values are used for identification

Common mistakes

  • Calculating Rf incorrectly (e.g. using 6.0/4.8 instead of 4.8/6.0)
  • Failing to mention that the single spot means the sample is pure
  • Not linking the Rf value to identification

Mark scheme (5 marks)

  1. Sample A is a pure substance because it produces only a single spot
  2. Sample B is a mixture because it produces more than one spot
  3. The Rf value of the spot in sample A is 0.8
  4. The Rf value can be used to identify the substance in sample A
  5. The substances in a mixture separate because they have different solubilities in the mobile phase / solvent, so they travel different distances up the paper

Key terms in this question

Rf value · paper chromatography

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