Question
Question: During a chromatography experiment, the solvent front travelled 8.0 cm from the baseline. A spot from substance X travelled 6.0 cm. (a) Calculate the Rf value of substance X. Give your answer to 2 decimal places. (2) (b) The chromatogram of an unknown sample Y produced three spots, one of which had the same Rf value as X. State and explain two conclusions you can draw about sample Y. (3)
Mark scheme (5 marks): (a) Rf calculation (2 marks):
- Rf = distance moved by substance ÷ distance moved by solvent = 6.0 ÷ 8.0 (1)
- = 0.75 (1) [accept 0.75 only; deduct if not to 2 d.p.]
(b) Conclusions about Y (3 marks):
- Y is a mixture (1) because it produced more than one spot / pure substances give a single spot (1)
- Y may contain substance X / X is possibly present (1) — accept: cannot be certain because two different substances can have the same Rf in one solvent; would need to repeat in a second solvent to confirm (1 — alternative mark)
5 marks · take your time before peeking.
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Generated by TopMyGrade AI · cross-check official sources before relying on the mark-scheme phrasing.