OCR A Level Chemistry A: Chromatography and qualitative analysis (H432) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions practise section 6.3.1, on chromatography and qualitative analysis, from the H432 Chemistry A specification from OCR (Version 3.1, May 2026, for first assessment in 2017). Together they test outcomes (a), (b)(i)–(ii) and (c)(i)–(vii), including calibration curves and the PAG6 and PAG7 techniques. H432 is the full A level. The final pair of questions are longer and combine several outcomes.

Study the guide and the notes first. Beyond this section, use the OCR chemistry hub, your printable checklist and the free diagnostics.

Questions

1. On a TLC plate the solvent front is 7.50 cm above the baseline. A spot has its centre 2.85 cm above the baseline. Calculate the Rf value of the spot. [2]

2. Describe a test-tube test that distinguishes 1-chlorohexane from 1-iodohexane. Give the reagent, the conditions and what you would see with each compound. [3]

3. A student oxidises a secondary alcohol to a ketone and follows the reaction by TLC on a silica plate. The solvent front is 8.00 cm above the baseline. The pure alcohol gives a spot at 2.24 cm and the pure ketone a spot at 5.36 cm. After 20 minutes the reaction mixture shows spots at 2.24 cm and 5.36 cm. After 60 minutes it shows a spot at 5.36 cm only.

(a) Calculate the Rf values of the alcohol and the ketone. [2] (b) State what the student can conclude from the plate after 60 minutes. [1] (c) Explain why the alcohol has the lower Rf value. [2]

4. A mixture of three alkanes is analysed by gas chromatography. Pure standards, run under the same conditions, give retention times of 2.1 min (hexane), 3.4 min (heptane) and 5.6 min (octane). The mixture gives peaks at 2.1, 3.4 and 5.6 min with integration values 4350, 2900 and 1450.

(a) State what is meant by retention time. [1] (b) Identify the component responsible for the peak at 3.4 min. [1] (c) Suggest why octane has the longest retention time. [2] (d) Calculate the percentage of each alkane in the mixture. [3]

5. A cleaning solution contains propan-2-ol. Standards give these GC peak areas.

Propan-2-ol / g dm⁻³	2.0	4.0	6.0	8.0	10.0
Peak area	3180	6420	9550	12800	15950

The cleaning solution is diluted by a factor of 10 and injected under the same conditions. Its propan-2-ol peak area is 7890.

(a) Plot a calibration graph and use it to find the concentration of propan-2-ol in the diluted sample. [3] (b) Calculate the concentration in the undiluted cleaning solution. [1] (c) Suggest why the solution was diluted before injection. [1] (d) State one condition that must be the same for the standards and the sample. [1]

6. Describe how Tollens’ reagent can distinguish butanal from butanone. Explain the result in terms of oxidation and reduction, including a half-equation. [4]

7. Bottle A contains 4-methylphenol, CH₃C₆H₄OH. Bottle B contains 2-methylpropanoic acid.

(a) Name a reagent that distinguishes A from B and give the observation for each. [2] (b) Give the equation when A is neutralised by sodium hydroxide solution. [1] (c) Write an equation for the reaction of 2-methylpropanoic acid with sodium carbonate. [1]

8. Compound X is a liquid carbonyl compound. It gives a precipitate with 2,4-DNP. Use these melting points for the 2,4-DNP derivatives of four candidates: F 98 °C, G 125 °C, H 142 °C, J 156 °C.

(a) Describe how the student should treat the precipitate before measuring its melting point. [2] (b) The treated derivative melts sharply at 141–142 °C. Identify X. [1] (c) State how the melting point would differ if the derivative were impure. [1]

9. Cyclohexene is shaken with bromine water. State what you would see and write an equation for the reaction. [2]

10. Six unlabelled bottles contain hex-1-ene, 1-iodobutane, pentanal, pentan-3-one, pentan-1-ol and propanoic acid. Plan a sequence of test-tube tests to identify every bottle. For each test, give the reagent, any conditions and the result that identifies a compound. [8]

11. A student heats propan-1-ol with acidified potassium dichromate and collects the distillate. GC of the distillate, with standards run under the same conditions, shows three peaks.

Component	propanal	propan-1-ol	propanoic acid
Peak area	2160	540	3300

(a) Calculate the percentage of each component, by peak area. [3] (b) The student wanted mainly propanal. Suggest what went wrong and how the method should change. [2] (c) Describe two test-tube tests that would confirm propanal and propanoic acid are both in the distillate. [2] (d) Explain why these percentages may not equal the true composition, and how a true concentration of propanal could be found. [1]

Answers

1. Rf = 2.85 ÷ 7.50, spot distance over front distance [1] = 0.38 [1] Examiner insight: An Rf above 1, or one given with units, means the division was inverted or mislabelled; check that your answer is a unitless number below 1.

2. Warm each with aqueous silver nitrate in ethanol [1] in a water bath [1]. 1-Iodohexane gives a yellow precipitate quickly; 1-chlorohexane gives a white precipitate much more slowly [1]. Examiner insight: Colours alone are not enough; the reagent and the warming are separate points, and “AgNO₃” with no solvent is weaker than the full reagent.

3. (a) Alcohol: 2.24 ÷ 8.00 = 0.28 [1]. Ketone: 5.36 ÷ 8.00 = 0.67 [1] (b) No alcohol spot remains, so the reaction is complete (no alcohol can be detected) [1]. (c) The alcohol has an O–H group and is more polar / can hydrogen bond to the silica [1], so it is adsorbed more strongly by the stationary phase and moves less far [1]. Examiner insight: In (c), name the interaction with the stationary phase; “it is heavier” or “it is less soluble” earns no credit.

4. (a) How long a component takes to pass through the column, from injection until it is detected [1]. (b) Heptane, as its retention time matches the standard [1]. (c) Octane has the highest boiling point / is least volatile [1], so it spends more time in (is more soluble in) the stationary phase [1]. (d) Total area = 4350 + 2900 + 1450 = 8700 [1]. Hexane = 4350 ÷ 8700 × 100 = 50.0% [1]. Heptane = 33.3% and octane = 16.7% [1] Examiner insight: Use the integration values given; percentages that do not add to about 100% signal a slip worth checking before moving on.

5. (a) All five points plotted correctly, area against concentration [1]; straight line of best fit [1]; reading at 7890 gives 4.9 g dm⁻³ (4.8–5.0) [1] (b) 4.9 × 10 = 49 g dm⁻³ [1] (c) The undiluted area would be far above the largest standard, so the reading would need extrapolation [1]. (d) Any one: same column, oven temperature, carrier gas flow rate or injection volume [1]. Examiner insight: Draw a long ruled best-fit line and mark the read-across from 7890 on the graph, so your reading is accurate and your method can be followed.

6. Add Tollens’ reagent to a sample of each and stand the tubes in a hot water bath [1]. Only the butanal tube develops a silver mirror; the butanone tube is unchanged [1]. Butanal is oxidised to butanoic acid: C₃H₇CHO + [O] → C₃H₇COOH [1]. Silver ions are reduced: Ag⁺ + e⁻ → Ag [1]. Examiner insight: Credit goes to naming both changes, butanal oxidised and silver ions reduced; writing only “the aldehyde reduces” leaves the silver half out.

7. (a) Sodium carbonate solution [1]. B gives effervescence (CO₂, turns limewater milky); A shows no effervescence [1]. (b) CH₃C₆H₄OH + NaOH → CH₃C₆H₄ONa + H₂O [1] (c) 2(CH₃)₂CHCOOH + Na₂CO₃ → 2(CH₃)₂CHCOONa + CO₂ + H₂O [1] Examiner insight: Sodium hydroxide cannot be the answer to (a) because both compounds react with it; pick a reagent that gives different results.

8. (a) Filter off the solid and recrystallise it (dissolve in the minimum of hot solvent, cool, filter) [1]; then dry it [1]. (b) H [1] (c) Lower, and over a wider range [1]. Examiner insight: “Purify it” alone is too vague for (a); name the technique and its steps.

9. Orange colour disappears / bromine water decolourises [1]. C₆H₁₀ + Br₂ → C₆H₁₀Br₂ [1] Examiner insight: “Goes clear” is not accepted for colourless; clear means transparent, and coloured solutions are also clear.

10. Use a fresh portion of each unknown for every test, in this order, testing only bottles not yet identified [1].

• Sodium carbonate solution: only propanoic acid effervesces, and the gas turns limewater milky [1].
• 2,4-DNP: yellow-orange precipitate with two bottles, pentanal and pentan-3-one [1].
• Tollens’, warm: silver mirror identifies pentanal; the other is pentan-3-one [1].
• Bromine water on the remaining three: hex-1-ene turns it from orange to colourless [1].
• Silver nitrate in ethanol, warm, on the last two: 1-iodobutane gives a yellow precipitate [1].
• Acidified dichromate, warm: the last bottle turns it from orange to green, confirming pentan-1-ol [1].
• Heat with a water bath, because the organic liquids ignite easily near a flame [1]. Examiner insight: Order matters: dichromate also turns green with pentanal, so a plan that uses it before the carbonyls are found cannot identify the alcohol.

11. (a) Total = 2160 + 540 + 3300 = 6000 [1]. Propanal 36.0%, propan-1-ol 9.0% [1]. Propanoic acid 55.0% [1] (b) Too much oxidation: excess oxidising agent and/or the aldehyde stayed in the hot mixture (reflux) [1]. Use excess propan-1-ol and distil the propanal off as it forms [1]. (c) Tollens’, warm: silver mirror shows propanal [1]. Sodium carbonate: effervescence shows propanoic acid [1]. (d) The detector may not respond equally to each compound; run propanal standards to make an external calibration curve [1]. Examiner insight: In (c) the test must identify the group asked for; acidified dichromate would not show propanal here, as propan-1-ol also turns it green.

Where marks are usually lost

• Measuring Rf distances from the plate edge or to the top of the spot.
• Giving GC proportions from peak heights, or quoting retention time as “how long the peak lasts”.
• Missing the dilution factor in a calibration question.
• Plotting calibration points carelessly or drawing a dot-to-dot line instead of a best-fit line.
• Naming a precipitate colour without the reagent or the warming step.
• Choosing a test that gives the same result for both compounds being compared.
• Writing that 2,4-DNP shows an aldehyde, or that a ketone gives a silver mirror.
• Leaving the organic product out of oxidation explanations, or not stating that silver ions are reduced.
• Describing heating with a Bunsen burner for flammable organic liquids.

Next steps

• Recap the tables and method boxes: revision notes
• Relearn anything you missed: study guide
• PAG skills in context: Practical skills for the Practical Endorsement
• The rest of the course: OCR chemistry hub and printable checklist
• Check other topics: all free 10-minute diagnostics
• Book a free trial class: /trial/

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 6.3.1 Chromatography and qualitative analysis, learning outcomes (a), (b)(i)–(ii) and (c)(i)–(vii), with their additional guidance, PAG6 and PAG7. Published by OCR.