Fig. 1 shows a thin-layer chromatography plate used by an environmental scientist. The scientist is comparing a pesticide reference, R, with water extracts ...
Question 1 Report
Fig. 1 shows a thin-layer chromatography plate used by an environmental scientist. The scientist is comparing a pesticide reference, R, with water extracts from two ponds, A and B. A solvent rises up the coated plate. The plate was removed when the solvent front was 7.5 cm above the baseline. Spot A1 travelled 4.5 cm. The scientist uses Rf values to compare compounds, where Rf = distance travelled by spot divided by distance travelled by solvent.
(a) Complete the calculation of the Rf value for A1.
Rf = 4.5 / 7.5 = ______ [1]
(b) Use Fig. 1 to suggest whether pond A contains the pesticide in reference R. [3]
(c) Give two reasons why Rf values should be compared only when the same solvent is used. [2]
(d) Suggest why pond B may contain a different compound from either R or A1. [4]
Answer Details
- (a) \[R_f=\frac{4.5}{7.5}=0.60\] [1 mark]
- (b) Yes, pond A may contain the pesticide. Pond A has a spot at the same height as reference R. Therefore the two spots have the same \(R_f\) value and may be the same compound. [3 marks]
- (c) \(R_f\) values should be compared using the same solvent because the solvent affects how well a compound dissolves and how far it travels. Different solvents can therefore give different \(R_f\) values for the same compound. Any two. [2 marks]
- (d) Pond B has one spot, but it is at a different height from the reference spot and at a different height from either spot in pond A. It therefore has a different \(R_f\) value and may be a different compound. [4 marks]
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