CCEA A Level Chemistry
CCEA A Level Chemistry 5.4 Chromatography Exam Questions
Free CCEA A Level Chemistry revision for 5.4 Chromatography: what you need to know, common mistakes and exam-style questions with answers. Covers paper chromatography including two-way, thin layer chromatography, gas-liquid chromatography, Rf values, retention times and percentage composition from GLC.
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About CCEA A Level Chemistry 5.4 Chromatography
Section 5.4 of Unit A2 2 covers separation and identification by chromatography. You need to describe and explain one-way and two-way paper chromatography, thin layer chromatography (TLC) and gas-liquid chromatography (GLC), both qualitatively and quantitatively. All chromatography depends on the distribution of components between a stationary phase and a mobile phase. You must calculate and interpret Rf values and use GLC data, including peak areas, to find the percentage composition of a mixture. Chromatography links to amino acids (5.9), where components may need a locating agent, and to chemistry in medicine (5.11), where TLC and GLC-MS are used to identify drugs and check purity.
What you need to know
- Principle Components distribute themselves between the stationary phase and the mobile phase. Components more soluble in the mobile phase, or less strongly adsorbed to the stationary phase, move further or faster.
- Paper and TLC Spot the mixture on a pencil origin line, place in a solvent below the line, and cover to keep the atmosphere saturated. TLC uses a thin layer of silica or alumina on a plate; it is faster and gives better separation than paper.
- Rf values Rf = distance moved by component / distance moved by solvent front. Colourless spots are located with a locating agent (for example ninhydrin for amino acids) or UV light.
- Two-way chromatography After running in one solvent, the paper is turned 90 degrees and run in a second solvent, separating components with similar Rf values in the first solvent.
- GLC An inert carrier gas (mobile phase) carries the vaporised sample through a column packed with a liquid on a solid support (stationary phase). Retention time identifies components; peak area is proportional to the amount, so percentage = peak area / total area x 100.
Common mistakes in 5.4 Chromatography
- Drawing the origin line in ink. Ink would separate too; use pencil.
- Placing the origin line below the solvent level. The spots would dissolve into the solvent.
- Measuring Rf to the edge of a spot. Measure to the centre of the spot.
- Using peak height for percentage composition in GLC. Use peak area.
- Saying the carrier gas reacts with the sample. It must be inert, such as nitrogen or helium.
Try three 5.4 Chromatography exam-style questions
1. A spot moved 3.9 cm and the solvent front 6.5 cm. Calculate the Rf value. (1 mark)
Show the mark scheme answer
3.9 / 6.5 = 0.60.
2. A GLC trace has peaks with areas 36, 84 and 120 units. Calculate the percentage of the second component. (2 marks)
Show the mark scheme answer
Total area = 240; 84 / 240 x 100 = 35%.
3. Explain why two-way chromatography might be needed to separate a mixture of amino acids. (2 marks)
Show the mark scheme answer
Some amino acids have similar Rf values in the first solvent so are not separated; a second solvent at 90 degrees gives different Rf values and separates them.
5.4 Chromatography FAQs
What does CCEA A Level Chemistry 5.4 Chromatography cover?
It covers paper (one-way and two-way), thin layer and gas-liquid chromatography, Rf values, retention times and percentage composition from GLC.
Which unit is 5.4 Chromatography examined in?
It is examined in Unit A2 2: Analytical, Transition Metals, Electrochemistry and Organic Nitrogen Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
What calculations come up in chromatography?
Rf values from chromatograms, and percentage composition from GLC peak areas. Measure distances to the centre of spots and use areas, not heights.
