A student used paper chromatography to find out which known food dyes are present in a mixture M. Fig. 13.1 shows the chromatogram. Lane M is the mixture, a...
Question 1 Report
A student used paper chromatography to find out which known food dyes are present in a mixture M. Fig. 13.1 shows the chromatogram. Lane M is the mixture, and lanes 1, 2 and 3 are three pure known dyes. The distance from the start line to the solvent front is 13.0 cm. Table 13.1 shows the distance each spot moved from the start line.
| spot | distance moved by spot / cm | Rf (spot ÷ 13.0) |
|---|---|---|
| dye 1 | 6.5 | |
| dye 2 | 9.75 | 0.75 |
| dye 3 | 13.0 |
(a) State why lane M produces two spots. [1]
(b) Complete Table 13.1 by calculating the Rf values for dye 1 and dye 3. Show your working. [2]
(c) The two spots in mixture M moved the same distances as dye 1 and dye 3. Use the chromatogram to identify which dyes are present in mixture M, and state whether dye 2 is present. [2]
(d) State why all the spots must be placed on the same start line. [1]
(e) Explain why the start line must be above the level of the solvent in the tank. [2]
(f) Describe how you would make the spots visible if the dyes had been colourless. [1]
(g) Give the name used for the solvent that moves up the paper. [1]
(h) State the Rf value of a spot that does not move from the start line. [1]
Answer Details
This question is paper chromatography used to identify dyes, including calculating Rf values. Rf compares how far a spot moves with how far the solvent moves.
(a) Lane M produces two spots because M is a mixture of two different substances (dyes), and each dye travels its own distance up the paper [1].
(b) \( R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}} \), with the solvent front at 13.0 cm.
| spot | distance moved / cm | Rf (spot ÷ 13.0) |
|---|---|---|
| dye 1 | 6.5 | 0.5 |
| dye 2 | 9.75 | 0.75 |
| dye 3 | 13.0 | 1.0 |
Dye 1: \( R_f = 6.5 \div 13.0 = 0.5 \) [1]. Dye 3: \( R_f = 13.0 \div 13.0 = 1.0 \) [1]. (Rf has no units and is always between 0 and 1.)
(c) The two spots in M moved the same distances as dye 1 and dye 3, so dye 1 and dye 3 are present in M [1]. Dye 2 is not present, because M has no spot at the height dye 2 reached [1].
(d) All spots must be placed on the same start line so that every spot travels for the same distance and the distances moved can be compared fairly from the same starting point [1].
(e) The start line must be above the level of the solvent because if it were below the solvent [1], the spots would dissolve straight into the solvent and wash away instead of rising up the paper and separating [1].
(f) If the dyes were colourless, make the spots visible by spraying with a locating agent or by viewing under ultraviolet light [1].
(g) The solvent that moves up the paper is called the mobile phase (the solvent) [1].
(h) A spot that does not move from the start line has \( R_f = 0 \) [1], because the distance it moved is zero.
Exam tip: always draw the start line in pencil above the solvent, and remember Rf = spot distance divided by solvent distance, never the other way round.
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