ChromatographyAQA A-Level Chemistry: Topic test

20 questions, 54 marks

AQA A-Level Chemistry

Chromatography topic test

Total 54 marks

Name

Class

Date

1
A mixture of two plant pigments, carotene and xanthophyll, is separated by column chromatography. A column is packed with silica gel, which has polar Si–OH groups on its surface, and a non-polar solvent is run down the column. Carotene is a non-polar hydrocarbon. Xanthophyll has polar –OH groups.
(a)
Which pigment emerges from the bottom of the column first, and why?
[1 mark]
AXanthophyll, because it forms hydrogen bonds with the silica
BCarotene, because it is less strongly retained by the silica and more soluble in the non-polar solvent
CXanthophyll, because it is more soluble in the non-polar solvent
DBoth emerge at the same time because they are carried by the same solvent
(b)
In column chromatography, what are the stationary phase and the moving phase?
[1 mark]
AA liquid and a gas respectively
BA liquid and a solid respectively
CA gas and a liquid respectively
DA solid and a liquid respectively
(c)
Explain why xanthophyll moves down the column more slowly than carotene.
[2 marks]
Total for question 1: 4 marks
2
A biochemist breaks down a small peptide and spots the mixture of amino acids onto a silica TLC plate, together with a spot of pure glycine as a standard. After development the solvent front has moved 9.0 cm from the baseline. The glycine spot is 3.6 cm from the baseline. The mixture gives two spots, at 3.6 cm and 5.4 cm from the baseline.
(a)
What is the Rf value of glycine in this experiment?
[1 mark]
A0.40
B0.60
C2.5
D3.6
(b)
Which statement about the spot at 5.4 cm is correct?
[1 mark]
AIt is glycine, because it is part of the same hydrolysis mixture
BIt has an Rf value of 0.54
CIt is held less strongly by the stationary phase than glycine is
DIt is held more strongly by the stationary phase than glycine is
(c)
Amino acids are colourless. Explain why the plate was sprayed with ninhydrin before the distances were measured.
[2 marks]
Total for question 2: 4 marks
3
A fuel laboratory analyses a hydrocarbon blend using gas chromatography. The column is packed with a solid coated with a liquid and is held at a high temperature, and helium is passed through it under pressure. Under the same conditions, pure hexane, heptane and octane have retention times of 2.4 min, 3.9 min and 5.8 min respectively. The blend gives three peaks, at retention times of 2.4 min, 5.8 min and 7.1 min.
(a)
Explain why the components of the blend leave the column at different times.
[3 marks]
(b)
State and explain what the retention times show about the composition of the blend. Describe how mass spectrometry could be used to identify the component with a retention time of 7.1 min.
[4 marks]
Total for question 3: 7 marks
4
A student wants to find out whether a pain-relief tablet contains paracetamol, aspirin or caffeine. She dissolves a crushed tablet in a solvent and uses thin-layer chromatography (TLC) on a silica plate, running the extract alongside pure samples of the three compounds. All three compounds are colourless. After development the solvent front has moved 8.0 cm from the baseline. The extract gives spots at 2.0 cm, 4.4 cm and 5.6 cm from the baseline. Under the same conditions, the Rf values of the pure compounds are 0.25 for paracetamol, 0.55 for aspirin and 0.45 for caffeine.
(a)
Describe how she should carry out the TLC experiment, including how the spots are made visible.
[6 marks]
(b)
Calculate the Rf values of the three spots from the extract. Use them to explain what the tablet contains, and suggest what should be done to identify any substance that cannot be identified.
[6 marks]
Total for question 4: 12 marks
5
A water-testing laboratory analyses an extract from a river sample using gas chromatography linked to mass spectrometry (GC-MS). The extract is injected into a heated column through which an inert gas flows under pressure.
(a)
What is the moving phase in gas chromatography?
[1 mark]
AA liquid solvent such as ethanol
BAn inert gas such as helium
CA solid coated with a liquid
DSilica gel
(b)
What is the role of the mass spectrometer in GC-MS?
[1 mark]
ATo separate the components of the mixture
BTo heat the column to a high temperature
CTo vaporise the sample
DTo identify each separated component from its molecular ion and fragmentation pattern
(c)
Suggest why raising the temperature of the column reduces the retention times of all the components.
[2 marks]
Total for question 5: 4 marks
6
A chemist follows the progress of an organic reaction by thin-layer chromatography (TLC). A pencil line is drawn near the bottom of a silica plate and spots of the reaction mixture, the starting material and the pure product are placed on it. The plate is developed in a covered tank containing a shallow layer of solvent.
(a)
Why is the baseline drawn in pencil rather than in ink?
[1 mark]
AGraphite does not dissolve in the solvent, so the line does not move up the plate
BPencil lines are easier to see on silica
CInk would react with the silica
DGraphite increases the Rf value of each spot
(b)
Why is the tank covered with a lid?
[1 mark]
ATo keep the temperature of the solvent above its boiling point
BTo react the solvent with oxygen in the air
CTo saturate the air with solvent vapour so that solvent does not evaporate from the plate and the solvent front rises evenly
DTo make the colourless spots visible
(c)
The chemist repeats the experiment with a different solvent and finds that the Rf value of the product changes. Explain why.
[2 marks]
Total for question 6: 4 marks
7
A food-safety laboratory tests a fruit drink for a banned dye using TLC. The drink extract and pure samples of two dyes, A and B, are spotted on a silica plate. After development the solvent front has moved 11.0 cm from the baseline. Dye A has moved 7.7 cm and dye B has moved 4.4 cm. The drink extract gives two spots, at 4.4 cm and 6.6 cm from the baseline.
(a)
Calculate the Rf values of dye A, dye B and the extract spot at 6.6 cm.
[3 marks]
(b)
Deduce which dye the drink may contain. Explain why this result is not conclusive and suggest how the laboratory could increase confidence in the identification.
[4 marks]
Total for question 7: 7 marks
8
A pharmaceutical chemist makes an ester, E, from a carboxylic acid, A. The crude product contains both E and unreacted A. The mixture is separated by column chromatography using silica gel and a non-polar solvent, and fractions are collected from the bottom of the column. Each fraction is analysed by gas chromatography linked to mass spectrometry (GC-MS). Under the same conditions, pure E gives a peak at a retention time of 4.2 min with a molecular ion peak at m/z = 116, and pure A gives a peak at 9.8 min with a molecular ion peak at m/z = 88. Fraction 1 gives one peak, at 4.2 min with m/z = 116. Fraction 2 gives two peaks, at 4.2 min and 9.8 min. Fraction 3 gives one peak, at 9.8 min with m/z = 88.
(a)
Describe and explain how E and A are separated by the column, and explain why E is collected before A.
[6 marks]
(b)
Evaluate the purity of each fraction using the GC-MS data, explaining why mass spectrometry makes the identification more reliable than retention time alone. State what the chemist should do with the fractions.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).