AQA A-Level Chemistry Paper 3, 2021: Question 2

4 marks · Easy difficulty · Practical Techniques & Data Analysis

Chromatography of Amino Acids This question assesses students' understanding of thin-layer chromatography (TLC) in identifying amino acids. It involves calculating an Rf value to identify an unknown amino acid using provided chromatogram data and reference values. Students must also explain how to visualise amino acids after chromatography and describe why amino acids have different Rf values based on their interactions with the stationary and mobile phases.

Question

Question text

02 The protein fibroin can be broken down into amino acids using an enzyme.

02.1 A student uses thin-layer chromatography (TLC) to identify these amino acids.

The student identifies two of the amino acids as alanine and serine.

Use Figure 3 to calculate the Rf value of the unknown amino acid.

Show your working.

Use your Rf value and Table 1 to identify the unknown amino acid.

[2 marks]

Figure 3 Table 1

Amino acid Rf value

tyrosine 0.25

glycine 0.34

valine 0.64

leucine 0.73

Rf value

Identity

02.2 The amino acids cannot be seen as they move during the experiment.

State how the amino acids can be made visible at the end of the experiment.

[1 mark]

02.3 State why each amino acid has a different Rf value.

[1 mark]

Mark scheme

Show the mark scheme

Question Answers Additional comments/Guidelines Mark

2.1 27 M1 some relevant working is needed to arrive 1

M1 = 0.34

80 at 0.325 - 0.35

M2 glycine

no ECF based on M1 1

2.2 use uv lamp or ninhydrin allow developing / locating agent / iodine 1

2.3 each amino acid has different (relative) affinity/attraction allow reference to different solubility in solvent 1

to/solubility in stationary and mobile phases OR different affinity for stationary phase

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.