Chromatography and Rf
Q1.
A student investigated food dyes using paper chromatography.
This is the method used.
1.       Put a spot of food colouring X on the start line.
2.       Put spots of four separate dyes, A, B, C and D, on the start line.
3.       Place the bottom of the paper in water and leave it for several minutes.
Figure 1 shows the apparatus the student used.
Figure 1
 
(a)     Write down two mistakes the student made in setting up the experiment and explain what problems one of the mistakes would cause.
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(2)
(b)     Another student set up the apparatus correctly.
Figure 2 shows the student’s results. The result for dye D is not shown.
Figure 2
 
Calculate the Rf value of dye A
Give your answer to two significant figures.
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Rf value = ______________________
(3)
(c)     Dye D has an Rf value of 0.80. Calculate the distance that dye D moved on the chromatography paper.
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Distance moved by dye D = ______________________
(1)
(d)     Explain how the different dyes in X are separated by paper chromatography.
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(4)
(e)     Flame emission spectroscopy can be used to analyse metal ions in solution.
Figure 3 gives the flame emission spectra of five metal ions, and of a mixture of two metal ions.
Figure 3
 
Use the spectra to identify the two metal ions in the mixture.
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(2)
(f)     Explain why a flame test could not be used to identify the two metal ions in the mixture.
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(2)
(g)     Two students tested a green compound X.
The students added water to compound X.
Compound X did not dissolve.
The students then added a solution of ethanoic acid to compound X.
A gas was produced which turned limewater milky.
Student A concluded that compound X was sodium carbonate.
Student B concluded that compound X was copper chloride.
Which student, if any, was correct?
Explain your reasoning.
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(4)
(Total 18 marks)