OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.16 Chromatography Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.16 Chromatography: what you need to know, common mistakes and exam-style questions with answers. Covers thin-layer, column and gas chromatography, stationary and mobile phases, Rf values, retention times and gas chromatography coupled with mass spectrometry.
Practise 3.3.16 Chromatography with past papers
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About OxfordAQA International A Level Chemistry 3.3.16 Chromatography
Chromatography is the final A2 organic topic and is examined in Unit 4. You need to explain how chromatography separates mixtures by the balance between solubility in the mobile phase and retention by the stationary phase, and describe thin-layer chromatography (TLC), column chromatography and gas chromatography (GC). You must calculate Rf values from TLC, use retention times from GC, and compare both with known standards. You also need to know how GC can be combined with mass spectrometry to identify components. This topic links to amino acids and proteins, where TLC with ninhydrin is used, and to organic analysis and synthesis.
What you need to know
- Principle Components separate because of their different balance between solubility in the moving phase and retention by the stationary phase. Components that are more soluble in the mobile phase move further or faster.
- Thin-layer chromatography The stationary phase is a thin layer of silica or alumina on a plate and the mobile phase is a solvent. Rf = distance moved by spot / distance moved by solvent; compare with standard Rf values in the same solvent.
- Column chromatography A column is packed with a solid such as silica in a solvent. The mixture moves down with the solvent and components are collected separately as they leave the column.
- Gas chromatography The mobile phase is an unreactive carrier gas and the stationary phase is a solid or a liquid on a solid in a long coiled column. Retention time is the time a component takes to pass through the column.
- GC-MS Gas chromatography separates the mixture and the mass spectrometer identifies each component as it leaves the column, by comparing spectra with a database.
Common mistakes in 3.3.16 Chromatography
- Measuring the spot distance to the top or bottom of the spot. Measure to the centre of the spot from the baseline.
- Comparing Rf values found with different solvents. Rf values depend on the solvent, so standards must use the same conditions.
- Saying retention time depends only on boiling point. It depends on the balance between the gas phase and retention by the stationary phase.
- Saying GC alone identifies a compound. It separates components; identification needs comparison with standards or a mass spectrometer.
Try three 3.3.16 Chromatography exam-style questions
1. A spot moved 2.7 cm and the solvent front moved 9.0 cm. Calculate the Rf value. (1 mark)
Show the mark scheme answer
2.7 / 9.0 = 0.30.
2. Explain why two substances have different retention times in gas chromatography. (2 marks)
Show the mark scheme answer
They have different solubility in the liquid stationary phase / different retention by the stationary phase; the one more strongly retained takes longer to pass through the column.
3. Explain the advantage of coupling GC with mass spectrometry. (2 marks)
Show the mark scheme answer
GC separates the components of a mixture; MS gives a spectrum for each component, so it can be identified by comparison with a database.
3.3.16 Chromatography FAQs
What does OxfordAQA International A Level Chemistry 3.3.16 Chromatography cover?
It covers thin-layer, column and gas chromatography, stationary and mobile phases, Rf values, retention times and gas chromatography coupled with mass spectrometry.
Which unit is 3.3.16 Chromatography examined in?
It is examined in Unit 4, an International A2 paper covering organic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
What should I learn about each type of chromatography?
For each method, know the stationary phase, the mobile phase, how results are measured (Rf or retention time) and how components are identified by comparison with standards. For GC also know how it is combined with mass spectrometry.
