Chromatography techniquesAQA A-Level Chemistry: Subtopic test

1. 1
   A student separates the dyes in a black ink by thin-layer chromatography (TLC). A line is drawn near the bottom of a plate coated with silica, spots of the ink and of three known dyes are placed on the line, and the plate is stood in a shallow depth of solvent in a covered beaker.

   (a)

   Why is the line on the plate drawn in pencil rather than in ink?

   [1 mark]

   • APencil is more soluble in the solvent than ink
   • BInk would react with the silica on the plate
   • CGraphite does not dissolve in the solvent, so the line does not run up the plate
   • DA pencil line is darker, so the spots are easier to see

   (b)

   A dye is much more soluble in the solvent and is retained only weakly by the silica. Compared with a dye that is retained strongly, how will it appear on the developed plate?

   [1 mark]

   • AIt will stay on the baseline
   • BIt will travel a shorter distance, with a lower Rf value
   • CIt will have an Rf value of exactly zero
   • DIt will travel further up the plate, with a higher Rf value

   (c)

   Explain why the different dyes in the ink travel different distances up the plate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a TLC experiment the solvent front moved 7.5 cm from the baseline. A pure dye P moved 4.5 cm from the baseline. A sample of an unknown ink, run on the same plate, gave two spots, at 4.5 cm and 1.8 cm from the baseline.

   (a)

   What is the Rf value of dye P?

   [1 mark]

   • A0.60
   • B1.67
   • C0.40
   • D0.24

   (b)

   Which conclusion is justified by the results?

   [1 mark]

   • AThe ink is a pure substance
   • BThe ink contains at least two substances, one of which may be dye P
   • CThe second spot is dye P
   • DDye P is retained more strongly than the substance in the second spot

   (c)

   Calculate the Rf value of the second spot in the ink and state, with a reason, which of the two substances in the ink is more strongly retained by the stationary phase.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A forensic chemist analyses a mixture of volatile organic compounds using gas chromatography linked to mass spectrometry (GC–MS). The column is packed with a solid coated with a liquid, and an inert gas is passed through it under pressure at high temperature. The mixture gives three peaks, with retention times of 2.1, 3.8 and 5.2 minutes. A standard of compound S, run under identical conditions, has a retention time of 3.8 minutes.

   (a)

   Explain how gas chromatography separates the components of the mixture and how mass spectrometry is used after the separation.

   [3 marks]

   (b)

   Use the data to suggest which peak may be compound S. Explain why this is not proof, and how the chemist could confirm the identity.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic laboratory uses chromatography to separate and identify the components of mixtures taken from crime scenes, such as inks, drugs and fire accelerants. Different techniques are chosen for different samples.

   (a)

   Compare thin-layer chromatography, column chromatography and gas chromatography. In your answer describe the stationary and moving phases in each, explain what determines how far or how fast a substance travels, and state how substances are identified.

   [6 marks]

   (b)

   A technician uses TLC to identify the colourless amino acids in a mixture. Describe how the experiment is carried out and the results analysed. In one run, a spot moved 3.6 cm and the solvent front moved 9.0 cm; calculate the Rf value of this spot.

   [6 marks]

   Total for question 4: 12 marks

End of questions