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2025 O Level Chem P1 Q5 — Chromatography Solution

Chromatography 2025 O Level Pure Chemistry · Paper 1 · Q5

Question

Which statement about Rᶠ values in paper chromatography is correct?

AThe Rᶠ value is calculated as (distance moved by the solvent) ÷ (distance moved by the spot).
BThe Rᶠ value of each spot is a number between 0 and 100.
CThe unit of the Rᶠ value is the centimetre.
DWhen the length of the paper is doubled, the Rᶠ value for a sample is unchanged.

Correct answer: D

Video Solution

Explanation

Answer: D. The \(R_f\) value is a property of the substance under the given conditions, and is independent of the paper's length. Doubling the paper just gives a longer solvent run, with the spot moving twice as far — the ratio stays the same.

A is wrong: \(R_f\) = (distance moved by the spot) ÷ (distance moved by the solvent), not the other way round.

B is wrong: \(R_f\) is a ratio between 0 and 1 (since the spot can't travel further than the solvent front).

C is wrong: as a ratio of two distances, \(R_f\) has no units — it's dimensionless.

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