In a chromatography experiment, the solvent front travels 8.0 cm. A substance travels 6.0 cm. (a) Calculate the Rf value. (b) A reference sample of substance X has Rf = 0.90 in the same solvent. Are they the same substance? (3 marks)
Hint: Rf = substance distance / solvent front distance. Compare Rf values — identical Rf in the same solvent suggests same substance.
+30 XP
1 markPure substance testAQA 8462 style
How can chromatography show that a substance is impure?
A The substance travels less than 1 cm up the plate
B More than one spot appears on the chromatogram
C The Rf value is greater than 1
D The solvent front reaches the top of the paper
3 marksFormulation explanationAQA 8462 style
Explain what is meant by a formulation, and give one example. State why the exact proportions of each component matter. (3 marks)
+30 XP
1 markMelting point purityAQA 8462 style
A sample of aspirin has a melting range of 135-142°C. Pure aspirin melts at exactly 138°C. What does this suggest?
A The sample is pure and of high quality
B The sample is impure — impurities lower and broaden the melting range
C The sample is a different substance from aspirin
D The melting point depends on the mass of the sample used