OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 10

1 mark · Medium difficulty · Multiple Choice

Determine the Rf value for the compound most strongly adsorbed onto the stationary phase using the given TLC chromatogram.

Practise this question

Question

A TLC chromatogram diagram showing a rectangular plate with an origin line near the bottom and a solvent front line near the top. Two spots are shown: one higher up near the solvent front, and one lower down closer to the origin. Below the diagram, multiple-choice options A (0.25), B (0.33), C (0.69), and D (0.78) are listed alongside the question asking for the Rf value of the compound most strongly adsorbed onto the stationary phase.
Question text

10 A TLC chromatogram for two compounds is shown below.

solvent front

origin

What is the Rf value for the compound that is most strongly adsorbed onto the stationary phase?

A 0.25

B 0.33

C 0.69

D 0.78

Your answer

[1]

Mark scheme

Show the mark scheme A table showing question number 10 with the correct answer A, worth 1 mark.

10 A 1 AO2.1

How to answer it

TLC Chromatography and Rf Calculation

📌 What this question tests

This question assesses your understanding of Thin-Layer Chromatography (TLC), specifically how the strength of adsorption to the stationary phase relates to the distance traveled by a compound, and your ability to calculate retention factor ( Rf ) values.

Question 10 Analysis

Identifying the correct response and mastering the underlying chromatographic principles

✅ Correct Answer: A (0.25)

The correct option is A. The compound most strongly adsorbed onto the stationary phase travels the shortest distance from the origin and therefore has the lowest Rf value (0.25).

Mark Awarded: 1 / 1 mark (AO2.1)

💡 Key Knowledge

  • Stationary Phase: Usually a polar solid like silica gel or alumina. Strongly adsorbs polar compounds via hydrogen bonding or dipole-dipole interactions.
  • Mobile Phase: A liquid solvent that carries components up the plate.
  • Adsorption vs. Solubility: Compounds that stick strongly to the stationary phase move less. Compounds that are more soluble in the mobile phase move further.

🧠 Exam Technique

  • Step 1: Read the question carefully. It asks for the compound that is most strongly adsorbed, not the one that traveled the furthest.
  • Step 2: Locate the spot closest to the origin (the lower spot on the chromatogram).
  • Step 3: If required to calculate, measure from the origin to the center of the spot, and divide by the distance from the origin to the solvent front. Spot lower down = smaller Rf value.

❌ Common Errors

  • Inverting the relationship: Many students confuse stationary phase retention with mobile phase solubility, selecting option D (0.78) because it represents the highest number, forgetting that a higher Rf means weaker adsorption to the stationary phase.
  • Measurement errors: Measuring from the bottom of the TLC plate instead of the origin pencil line.

Topics

Module 6: Organic chemistry and analysis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.