Edexcel A-Level Chemistry Paper 2, June 2018: Question 3
11 marks · Medium difficulty · Practical Techniques and Data Analysis
Analyze thin layer chromatography and gas chromatography of amino acids and organic mixtures, calculating Rf values and sketching chromatograms.
Practise this questionQuestion
Question text
3 Chromatography is a technique used to separate the components of a mixture.
(a) A sample of a tripeptide was hydrolysed and then placed on a thin layer chromatography
(TLC) plate. Samples of possible amino acids present were also placed on the TLC plate
for reference.
A simplified diagram of the developed TLC plate is shown.
solvent front
origin
A B C D E F G H
A – Lysine B – Serine C – Histidine D – Tyrosine
E – Isoleucine F – Methionine G – Proline H – Hydrolysed tripeptide
(i) Calculate the Rf value for the amino acid lysine.
Give your answer to an appropriate number of significant figures.
(1)
(ii) Identify by name the two amino acids present in the tripeptide, giving a
reason for the lack of a third spot.
(3)
(iii)Givetworeasons why different amino acids have different R*P52293A0524*fvalues.
(2)
(iv) In chromatography, a ‘locating’ reagent is often used when the components in
a mixture are colourless.
Which reagent is used to locate the amino acid spots?
(1)
A iodine
B methyl orange
C ninhydrin
D phenolphthalein
(b) Gas chromatography can be used both to separate the components in a mixture
and to determine the amount of each present.
(i) State why argon and nitrogen are suitable carrier gases for gas chromatography.
(1)
(ii) A mixture containing one part substance X, two parts substance Y and one
part substance Z was separated by gas chromatography.
Substance X has a retention time of 10 seconds, substance Y of 15 seconds
and substance Z of 40 seconds.
Complete the sketch of this chromatogram.
(3)
Detector
current *P52293A0624*
10 20 30 40 50 60
Time/s
(Total for Question 3 = 11 marks)
Mark scheme
Show the mark scheme
Question
Acceptable Answer Additional guidance Mark
Number
3(a)(i) Example of calculation: (1)
Answer to 2 SF Rf=
1.5 = 0.15
Allow
0.14 – 0.16
Do not award 3SF, e.g.
0.140/0.150/0.160
Do not award an answer with units
Question
Acceptable Answer Additional guidance Mark
Number
3(a)(ii) An answer that makes reference to the following: (3)
serine (1)
methionine (1) Allow for 1 mark out of the first two for
F and B
(reason) one amino acid is present twice (in the Allow
tripeptide) ‘there are two serine amino acids/
there are two methionine amino acids’
OR
Another amino acid has the same Rf value as Do not award if given with any other
either serine or methionine (1) amino acid stated in the question
Ignore reference to another amino acid
not given in the table
Question
Acceptable Answer Additional guidance Mark
Number
3(a)(iii) An answer that makes reference to the following: Allow reverse arguments (2)
amino acids have different solubility / adsorption Do not award react with the stationary
to the stationary phase (1) phase
Allow “TLC plate” for stationary phase
Allow interact with/affinity for/form
different intermolecular forces with the
stationary or mobile phase
amino acids have different solubility in the mobile Allow “solvent” for mobile phase
phase (1) Ignore references to molecular
mass/size
Question
Answer Mark
Number
3(a)(iv) The only correct answer is C (1)
A is incorrect because this is a test for starch
B is incorrect because this is a strong acid-weak base indicator
D is incorrect because this is a weak acid-strong base indicator
Question
Acceptable Answer Additional guidance Mark
Number
3(b)(i) A statement that makes reference to (1)
gases are inert / do not react Ignore references to intermolecular
(with the components of the mixture) bonding / stability
Do not award for not an oxidising agent
Question
Acceptable Answer Additional guidance Mark
Number
3(b)(ii) A sketch that includes Penalise additional peaks (3)
Max 1 for the differences in height if
both peaks are not at correct positions
peak at 15 seconds and 40 seconds (1)
(peak at 15 seconds) with height at approximately
twice that of the peak at 10 seconds (1)
(peak at 40 seconds with) height at approximately
the same height as that of the peak at 10 seconds
(1)
Exemplar sketch
(Total for Question 3 = 11 marks)
How to answer it
Chromatography Analysis Study Guide
What this question tests
This question assesses your understanding of both thin-layer chromatography (TLC) and gas chromatography (GC). Key skills include calculating Rf values with correct significant figures, interpreting chromatogram spots to deduce peptide composition, explaining the principles of chromatographic separation (stationary vs. mobile phase interactions), identifying locating agents, knowing carrier gas properties, and sketching gas chromatograms with correct retention times and proportional peak heights.
Calculating the Rf Value for Lysine
✅ Correct Answer
0.15 (Acceptable range: 0.14 - 0.16 )
📐 Calculation Steps
- Measure the distance travelled by the spot from the origin (Lysine A is at 1.5 cm).
- Measure the distance travelled by the solvent front from the origin (10.0 cm).
- Apply the formula: Rf = (distance travelled by spot) ÷ (distance travelled by solvent front).
- Calculation: 1.5 ÷ 10 = 0.15
❌ Common Errors & Examiner Traps
- Wrong Significant Figures: Giving the answer to 3 SF ( 0.150 ) loses the mark. The question explicitly asks for an "appropriate number of significant figures" (2 SF based on ruler precision).
- Including Units: Rf is a ratio and has no units. Adding cm loses the mark.
Identifying Amino Acids in the Tripeptide
✅ Correct Answer
- Serine and Methionine
- Reason: One amino acid is present twice in the tripeptide (or another amino acid has the same Rf value as either serine or methionine).
🧠 Exam Technique
Compare the spots in column H (hydrolysed tripeptide) directly against the reference lanes A through G . Count the spots in column H and match their horizontal levels (Rf values) to the named standards.
❌ Common Errors
Students often fail to explain why there are only two spots visible for a tripeptide (which should yield three amino acid residues upon complete hydrolysis). You must explicitly state that one amino acid repeats within the sequence.
Reasons for Different Rf Values
💡 Key Knowledge
- Amino acids have different solubility / adsorption to the stationary phase (TLC plate).
- Amino acids have different solubility in the mobile phase (solvent).
- Alternatively: Different relative affinity or different intermolecular forces formed with the stationary/mobile phases.
❌ Common Misconceptions
Do not say amino acids "react" with the stationary phase. They form intermolecular forces (such as hydrogen bonding or van der Waals interactions), they do not undergo chemical reactions. Molecular mass/size references alone are also insufficient.
Locating Reagent for Amino Acids
✅ Correct Answer
C - ninhydrin
💡 Distractor Breakdown
- A (iodine): Used as a locating agent for alkenes/lipids, or a test for starch.
- B (methyl orange): Strong acid-weak base indicator.
- D (phenolphthalein): Weak acid-strong base indicator.
Carrier Gases in Gas Chromatography
✅ Correct Answer
The gases are inert / do not react with the components of the mixture.
🧠 Examiner Guidance
Keep your answer concise. Simply stating "inert" or "unreactive" secures the mark. Ignore references to intermolecular bonding, stability, or general gas properties unless tied directly to chemical inertness.
Sketching a Gas Chromatogram
✅ Correct Answer & Key Features
- Peak positions: New peaks must be drawn precisely at retention times 15 s (Substance Y) and 40 s (Substance Z).
- Peak height for Y (15 s): Must be approximately twice the height of the reference peak for X (since Y has a proportion of "two parts").
- Peak height for Z (40 s): Must be approximately the same height as the reference peak for X (one part).
🧠 Drawing Strategy
Check the ratio given in the stem: 1 part X, 2 parts Y, 1 part Z. Peak height/area is directly proportional to the amount of substance present. Scale your peak heights relative to the pre-drawn peak at 10 s (X).
Topics
Organic Chemistry · Core Practicals · Core Practical 15: Analyse organic and inorganic unknowns · Topic 19: Modern Analytical Techniques II · Topic 18: Organic Chemistry III
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.