QUESTION 153 (5 marks)
An amino-acid mixture is compared with three standards by TLC in two different solvent systems. In system 1 the solvent front travels 8.0 cm; the mixture has spots at 2.4 cm and 5.6 cm. In system 2 the mixture has Rf values 0.50 and 0.60. All standard comparisons are made under the same conditions within each solvent system.
| Standard | Rf, system 1 | Rf, system 2 |
|---|---|---|
| A | 0.30 | 0.20 |
| B | 0.30 | 0.50 |
| C | 0.70 | 0.60 |
(a) Calculate the mixture’s two Rf values in system 1. [2 marks]
(b) Identify the two standards most consistent with the mixture, using both systems. Explain. [2 marks]
(c) Explain why a standard’s Rf need not be the same in the two solvent systems. [1 marks]
Practice marking scheme
Answer
System 1 Rf values are 0.30 and 0.70. The evidence supports B and C. Rf changes when relative interactions with the mobile and stationary phases change.
Working
Divide 2.4 and 5.6 by 8.0. System 1 cannot distinguish A from B because both have Rf = 0.30. System 2 places the mixture at B’s 0.50 and C’s 0.60, resolving the A/B ambiguity. A change in solvent changes amino-acid solubility/intermolecular interactions with the mobile phase relative to the stationary phase, so Rf is not an intrinsic constant independent of conditions. Matching two systems supports identity without proving it absolutely.
Marking criteria
- Calculates Rf = 0.30 for the lower system-1 spot. [1 mark]
- Calculates Rf = 0.70 for the upper system-1 spot. [1 mark]
- Identifies B and C as the supported components. [1 mark]
- Explains that system 2 resolves the A/B co-migration in system 1. [1 mark]
- Explains Rf dependence on relative mobile/stationary phase interactions and solvent conditions. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.