Paper chromatography
4 marks · NormalThe chromatogram below shows the results of a chromatography experiment. The solvent front travelled 7.5 cm from the pencil line. There are three spots on the same chromatogram, labelled X, Y, and Z. The distances from the pencil line to the centres of the spots are:
| Spot | Distance from pencil line (cm) |
|---|---|
| X | 1.9 |
| Y | 4.2 |
| Z | 6.0 |
Calculate the Rf value for spot Y. Show your working.
Which spot is most soluble in the solvent? Give a reason.
Explain how you could use Rf values to identify a dye in the mixture.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 0.56
(b) Spot Z; it travelled furthest up the paper, so is most soluble.
(c) Compare the Rf value of the spot to known Rf values for dyes using the same solvent and paper; a match suggests identity.
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct formula and substitution for Rf value using spot Y: 4.2/7.5.
A1 (a) · 1 mark · Accurately calculates Rf = 0.56 (to 2 decimal places, 0.01 tolerance).
B2 (b) · 1 mark · Identifies Z as most soluble and explains it travelled furthest up the paper.
B3 (c) · 1 mark · Explains how to identify a dye by matching its Rf value with that of a known dye using the same conditions.
Worked solution
a) Rf value for spot Y = (distance moved by Y) / (distance moved by solvent front)
b) Spot Z is most soluble; it travelled furthest up the paper, so it dissolved best in the solvent.
c) To identify a dye, compare its Rf value with known Rf values. If the Rf matches a known dye (chromatographed under identical conditions), the dye is likely the same.