8.2 Paper chromatography and Rf values
5 exam questions worth 7 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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In paper chromatography, what is the stationary phase?
Choose one answer.
Correct Not quite: the answer is The paper
Mark scheme
C
Model answer
C: the paper does not move; the solvent is the mobile phase.
A spot moved 3.6 cm. The solvent front moved 9.0 cm.
Calculate the Rf value.
Mark scheme
- 3.6 ÷ 9.0 (1)
- = 0.40 (1)
Model answer
Rf = 3.6 ÷ 9.0 = 0.40
An ink gives three spots on a chromatogram. What does this tell you about the ink?
Mark scheme
- it is a mixture of (at least) three substances (1)
Model answer
The ink is a mixture of at least three different substances.
Explain why different substances in a mixture travel different distances up the paper.
Mark scheme
- they have different solubilities in the solvent (1)
- and different attractions to the paper (1)
Model answer
Each substance has a different solubility in the solvent and a different attraction to the paper, so they move at different speeds.
Explain why an Rf value can never be greater than 1.
Mark scheme
- a substance cannot move further than the solvent (1)
Model answer
The substance is carried by the solvent, so it can never travel further than the solvent front.
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