OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 13

1 mark · Medium difficulty · Multiple Choice

Interpret a gas chromatogram of a three-component mixture to identify true statements regarding retention time, peak area proportions, and composition.

Practise this question

Question

A multiple choice question featuring a gas chromatogram showing recorder response against time in minutes for components X, Y, and Z. A box inset gives the relative peak areas as X = 1.0, Y = 3.5, and Z = 5.5. Three numbered statements follow about the chromatogram, and four options A to D are provided.
Question text

13 A sample containing a mixture of 3 components, X, Y and Z, is analysed using gas

chromatography.

The gas chromatogram below is obtained.

The relative peak areas of X, Y and Z are included.

Z Relative peak areas

X = 1.0

1000 Y Y = 3.5

Z = 5.5

Recorder response X

05 10 15

Time/min

Which statement(s) is/are true?

1 The peak for component X shows the mass of one mole.

2 Component Y stays in the column for longer than component X.

3 Component Z consists of more than half of the sample.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer for question 13 is C.

13 C 1

How to answer it

Interpreting Gas Chromatograms

What this question tests

This question assesses your ability to interpret gas chromatography (GC) data, specifically understanding retention times (x-axis), peak areas and their proportional relationship to component concentrations/amounts, and the fundamentals of chromatographic separation.

Question 13 — Multiple Choice

Analyzing Statements 1, 2, and 3

✅ Correct Answer: C (Only 2 and 3)

Statement 1 is incorrect, while Statements 2 and 3 are correct. Therefore, option C is the right choice.

💡 Key Knowledge: Peak Areas vs. Mass

  • Retention Time: The time taken for a component to travel through the column (read on the x-axis). Longer retention time means it stays in the column longer.
  • Peak Area: Directly proportional to the concentration or amount of that component in the sample mixture, not its molar mass.

🧠 Exam Technique

Evaluate each statement individually using a true/false checklist before looking at the final options A–D. Eliminate options systematically to save time and prevent misreading.

📐 Step-by-Step Statement Breakdown

  1. Statement 1: False . The peak area represents relative abundance/amount, not the absolute mass of "one mole".
  2. Statement 2: True . Component Y has a retention time of approximately 4 minutes, whereas X has a retention time of about 1.5 minutes. Y takes longer to travel through the column.
  3. Statement 3: True . Total peak area = 1.0 (X) + 3.5 (Y) + 5.5 (Z) = 10.0. Component Z has an area of 5.5, which is greater than half of the total (5.5 / 10.0 = 55%).

❌ Common Errors

Students frequently fall for the trap in Statement 1 by incorrectly linking peak height or area to molecular mass or moles, confusing quantitative chromatography calibration with stoichiometry.

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), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.