Chromatography and Rf
Q1.
A student investigated the colours in three different flowers, A, B and C, using paper chromatography.
The colours are soluble in ethanol but are insoluble in water.
This is the method used.
1.   Place ethanol in a beaker.
2.   Add the flower.
3.   Stir until the colours dissolve in the ethanol.
4.   Filter the mixture.
5.   Put spots of the coloured filtrate on the chromatography paper.
(a)  The filtrate was a very pale coloured solution.
How could the student obtain a darker coloured solution?
Tick two boxes.
 
Crush the flower		
Filter the mixture three times		
Use a larger beaker		
Use more ethanol		
Use more flowers		
(2)
(b)  Figure 1 shows the apparatus used.
Figure 1
 
What two mistakes did the student make in setting up the apparatus?
Tick two boxes.
 
The paper does not touch the beaker		
The start line is drawn in ink		
The water level is below the start line		
Uses a lid on the beaker		
Uses water as the solvent		
(2)
(c)  Another student sets up the apparatus correctly.
Figure 2 represents the student’s results.
Figure 2
 
What two conclusions can be made from Figure 2?
Tick two boxes.
 
Flower A contains a single pure colour		
Flowers A and B contain the same colours		
The colour in flower C is a mixture		
The colour in flower B was the least soluble		
Two of the colours have the same Rf value		
(2)
(d)  The student records some measurements.
The measurements are:
•   the colour from flower B moves 7.2 cm
•   the solvent moves 9.0 cm
Calculate the Rf value for the colour from flower B.
Use the equation:
 
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(2)
(Total 8 marks)