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CAIE A Level Chemistry 9701 · Analytical techniques

Analytical techniques: describe question

Describe7 marksChallenge

In an experiment using thin-layer chromatography, a mixture of three compounds A, B and C is separated. The stationary phase is aluminium oxide and the mobile phase is a non-polar solvent. The measured Rf values are: A = 0.75, B = 0.45, C = 0.20.

(a) Define the following terms as used in this technique: (i) stationary phase (ii) mobile phase (iii) baseline (iv) solvent front (v) Rf value

(b) Explain why compound A has a higher Rf value than compound C. Refer to interactions with the stationary phase and solubility in the mobile phase in your answer.

0 words · 7 marks

No account needed for your first marks. Your answer is used only to mark it.

How this question is marked

7 marks, so the mark scheme is looking for about 7 separate creditworthy points. Making the same point twice in different words earns it once.

Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.

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