Paper chromatography
2 marks · NormalState what is meant by the term "Rf value" in paper chromatography, and explain why the Rf value is always less than 1.
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Also accept: Rf value is the ratio of the distance moved by the substance to the distance moved by the solvent front, measured from the origin/pencil line; it is always less than 1 because the substance cannot move further than the solvent front.
Common mistakes: Rf value has units; Rf can be more than 1
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Marking points
B1 · 1 mark · Defines Rf as (distance moved by spot)/(distance moved by solvent front), both measured from the pencil line.
B2 · 1 mark · Explains Rf value is always less than 1 because the spot cannot move further than the solvent front.
Worked solution
The Rf value in paper chromatography is the ratio of the distance moved by the spot (measured from the pencil line) to the distance moved by the solvent front (measured from the same line).
The Rf value is always less than 1 because the spot cannot travel further than the solvent front; it always stays behind or at the most moves with the solvent, but never beyond it.