A student uses thin-layer chromatography (TLC) to identify these amino acids - AQA - A-Level Chemistry - Question 2 - 2021 - Paper 3
Question 2
A student uses thin-layer chromatography (TLC) to identify these amino acids.
Use Figure 3 to calculate the R value of the unknown amino acid.
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Worked Solution & Example Answer:A student uses thin-layer chromatography (TLC) to identify these amino acids - AQA - A-Level Chemistry - Question 2 - 2021 - Paper 3
Step 1
Calculate the R value of the unknown amino acid
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Answer
To find the R value of the unknown amino acid, we use the formula:
R=dsda
where:
da is the distance moved by the amino acid (unknown amino acid in this case).
ds is the distance moved by the solvent front.
Based on Figure 3, if we assume the unknown amino acid moved to a distance of 0.34 cm and the solvent front moved to a distance of 1.0 cm, we calculate:
R=1.00.34=0.34
Therefore, the R value of the unknown amino acid is 0.34.
Step 2
Identity
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Answer
Referring to Table 1, an R value of 0.34 corresponds to the amino acid glycine. Therefore, the identity of the unknown amino acid is glycine.
Step 3
State how the amino acids can be made visible at the end of the experiment.
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Answer
The amino acids cannot be seen as they move during the experiment. They can be made visible at the end using a UV lamp or by applying ninhydrin, which reacts with amino acids to produce a colored compound.
Step 4
State why each amino acid has a different R value.
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Answer
Each amino acid has different (relative) affinity/attraction to the stationary and mobile phases, which results in varying solubility in the solvent during chromatography.
