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Using Rf to find solvent distance
The blue colour moved 6.4 cm up the chromatogram.
The Rf value of the blue colour is 0.87.
Calculate the distance moved by the solvent. Give your answer to 2 significant figures.
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Marking points
- 1 Uses the Rf relationship with 0.87, 6.4 cm and the unknown solvent distance in the correct positions.
- 2 Rearranges to divide 6.4 by 0.87.
- 3 Obtains the unrounded value 7.356 (or the corresponding calculator value).
- 4 Gives 7.4 cm to 2 significant figures.
Full-mark answer
Rf = distance moved by colour ÷ distance moved by solvent, so 0.87 = 6.4 ÷ d. Therefore d = 6.4 ÷ 0.87 = 7.356… cm, which is 7.4 cm to 2 significant figures.
Why this answer loses marks
Rf is the solvent distance divided by the colour distance.
The ratio is inverted; start with colour distance divided by solvent distance before rearranging.