A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Chromatography Techniques in Organic Chemistry — mark scheme explained
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.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the given values.
- S2
Distance traveled by the solvent front (d s ) = 5 cm
- S3
Distance traveled by the substance (d c ) = 2 cm
- S4
Step 2: Use the formula for R f value.
- S5
R f = d c / d s
- S6
Step 3: Substitute the values into the formula.
- S7
R f = 2 cm / 5 cm
- S8
Step 4: Perform the division.
- S9
R f = 0.4
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the given values.
- S2
Distance traveled by the solvent front (d s ) = 5 cm
- S3
Distance traveled by the substance (d c ) = 2 cm
- S4
Step 2: Use the formula for R f value.
- S5
R f = d c / d s
- S6
Step 3: Substitute the values into the formula.
- S7
R f = 2 cm / 5 cm
- S8
Step 4: Perform the division.
- S9
R f = 0.4
Final answer: 0.4
Common mistakes
- Using the wrong distances to calculate R f values. — 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. — Always use the formula R f = distance traveled by the substance / distance traveled by the solvent front.
- Confusing retention times with R f values. — 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. — 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 ). — 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. — 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
- Full worked solution (all marking points)3 marks