Chromatography and purityIB MYP Chemistry: Subtopic test

1. 1
   A student uses paper chromatography to analyse a black ink. The solvent front moves 8.0 cm from the start line. The ink separates into three spots, which move 2.0 cm, 4.8 cm and 6.4 cm from the start line. She runs three known dyes on the same paper: dye A moves 2.0 cm, dye B moves 3.2 cm and dye C moves 6.4 cm.

   (a)

   Which known dyes are present in the black ink?

   [1 mark]

   • AA and B
   • BB and C
   • CA and C
   • DA, B and C

   (b)

   What is the Rf value of dye B?

   [1 mark]

   • A0.40
   • B2.5
   • C0.60
   • D0.25

   (c)

   Explain how the chromatogram shows that the ink is not a pure substance.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student sets up paper chromatography for a sample of green food colouring. She draws a start line near the bottom of the paper in pencil, puts a small spot of the sample on the line and stands the paper in a beaker containing a shallow layer of solvent. The solvent level is below the start line, and she puts a lid on the beaker.

   (a)

   Why does she draw the start line in pencil and not in ink?

   [1 mark]

   • APencil dries more quickly than ink
   • BPencil does not dissolve in the solvent and move up the paper
   • CPencil is more soluble than the sample
   • DPencil makes the sample move faster

   (b)

   Why must the solvent level be below the start line?

   [1 mark]

   • ASo that the paper does not become wet
   • BSo that the solvent can evaporate more quickly
   • CSo that the lid can be fitted
   • DSo that the sample does not dissolve into the solvent in the beaker

   (c)

   Explain how the different colours in the food colouring become separated.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student tests the purity of two samples of a white solid sold as pure benzoic acid, which has a melting point of 122 °C. She heats a small amount of each sample slowly in a melting point tube and records the range of temperatures over which it melts. Sample 1 melts between 121 °C and 122 °C. Sample 2 melts between 112 °C and 119 °C.

   (a)

   Decide which sample is purer, using the data to justify your answer.

   [3 marks]

   (b)

   Suggest improvements to make the investigation more reliable, and state one safety precaution.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic scientist investigates a threatening note written in black ink. She runs paper chromatography on ink from the note and from the pens of three suspects. In each run the solvent front moves 10.0 cm from the start line. The ink from the note separates into two spots at 3.0 cm and 7.5 cm. Pen 1 gives spots at 3.0 cm and 6.0 cm, pen 2 gives spots at 3.0 cm and 7.5 cm, and pen 3 gives spots at 2.5 cm and 7.5 cm. The police are considering using the result as evidence against the owner of one of the pens.

   (a)

   Calculate the Rf values for the ink from the note and decide which pen it matches, justifying your decision.

   [6 marks]

   (b)

   Discuss and evaluate the use of chromatography results as evidence against a suspect.

   [6 marks]

   Total for question 4: 12 marks

End of questions