A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: Chromatography Techniques in Organic Chemistry — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 3 July 2026

The short answer

Chromatography is a powerful analytical technique used to separate and identify components in a mixture. It plays a crucial role in organic chemistry, particularly for the purification and analysis of complex mixtures. This section covers three main types of chromatography: thin-layer chromatography (TLC), column chromatography (CC), and gas chromatography (GC).

The question

A TLC plate was developed using a solvent. The distance traveled by the solvent front (d s ) is 5 cm. A spot for an unknown compound is found at a distance of 2 cm from the origin. Calculate the R f value for this compound.

[Paraphrased for study — not reproduced from any exam paper.]

3 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.

What the command words demand

Calculate: Perform a numerical calculation using given data and appropriate formulas.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Determine: Find or derive a specific value or quantity based on given information.
Analyze: Examine and interpret data or graphs to draw conclusions about the behavior of the system.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.

Identify the given values.0 marks
Distance traveled by the solvent front (d s ) = 5 cmDistance traveled by the substance (d c ) = 2 cm
Use the formula for R f value.1 mark
R f = d c / d s
Substitute the values into the formula.1 mark
R f = 2 cm / 5 cm
Perform the division.1 mark
R f = 0.4

Final answer: 0.4

Work through every step correctly and you earn all 3 marks.

Another worked example

A gas chromatogram shows that a compound elutes at a retention time of 12 minutes. A known standard has a retention time of 12.5 minutes, measured under the same conditions. Comment on whether the unknown could be the same compound as the standard.

3 marks

Identify the given values.0 marks
Retention time of the unknown compound = 12 minutesRetention time of the known standard = 12.5 minutes
Compare the two retention times.1 mark
The retention times (12 min vs 12.5 min) are close but not identical, differing by 0.5 minutes.
Consider the conditions.1 mark
Retention times are only reliable for identification when measured under identical conditions (same column, temperature, carrier gas flow rate, etc.).
Draw a conclusion.1 mark
The values are similar, so the unknown could be the same compound, but the small difference means identification cannot be confirmed from retention time alone; the conditions must be kept identical and ideally GC-MS used for confirmation.

Final answer: The retention times (12 min vs 12.5 min) are close but not identical. As retention times are only reliable for identification when measured under identical conditions, this small difference means the unknown may be the same compound, but identification cannot be confirmed from retention time alone.

Work through every step correctly and you earn all 3 marks.

Common mistakes

Using the wrong distances to calculate R f values.
Why it happens: Students may measure the distance from the origin to the edge of the spot instead of the center, leading to incorrect calculations.
Fix: Always measure the distance from the origin to the center of the spot for accurate R f values.
Forgetting to use the correct formula for R f values.
Why it happens: The formula for R f can be easily overlooked, leading to incorrect calculations.
Fix: Always use the formula R f = distance traveled by the substance / distance traveled by the solvent front.
Confusing retention times with R f values.
Why it happens: Students sometimes mix up the concepts of retention times (used in GC) and R f values (used in TLC), leading to incorrect answers.
Fix: Remember that retention times are used in GC, while R f values are used in TLC. Retention times measure time, while R f values measure distance.
Misinterpreting chromatograms.
Why it happens: Students may not fully understand how to read and interpret chromatograms, leading to incorrect identification of substances.
Fix: Practice analyzing chromatograms by identifying the solvent front, measuring distances accurately, and comparing with known standards. Ensure you understand the principles of separation in each technique.
Incorrectly calculating relative retention times (R r ).
Why it happens: Students may use the wrong formula or make arithmetic errors when calculating R r = retention time of the unknown / retention time of the standard.
Fix: Always use the correct formula for relative retention time and double-check your calculations to ensure accuracy.
Failing to explain the principles of separation clearly.
Why it happens: Students may struggle to articulate the principles of chromatography, leading to vague or incorrect explanations.
Fix: Practice explaining the principles step-by-step. Emphasize how substances are separated based on their interactions with the mobile and stationary phases in each technique (TLC, CC, GC).

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Calculate Rf Value	Use the Rf formula to divide the compound distance by the solvent front distance.	3
Retention Time Comparison	Compare retention times to judge whether an unknown matches a known standard.	3
Calculate Rf Values	Divide each spot's distance by the solvent front distance to find both Rf values.	4
Chromatography Identification	Compare retention times to judge whether the unknown compound matches the known standard.	4
Calculate Rf Values	Use the Rf formula to calculate retention factors for three spots on a TLC plate.	6
Total across these question types		20

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.