Edexcel A-Level Chemistry Paper 3, November 2021: Question 6

21 marks · Hard difficulty · Open Response

Analyze the multi-step preparation and purity analysis of paracetamol including phenol nitration, intermolecular forces, filtration under reduced pressure, recrystallisation, and a redox titration calculation.

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Question

Question text

6 This question is about the preparation and analysis of paracetamol.

H O

HO N C CH3

paracetamol

Paracetamol may be prepared from phenol in three stages.

NO2

Stage 1

HO HO NO2 HO

4-nitrophenol 2-nitrophenol

Stage 2

HO NO2 HO NH2

4-aminophenol

H O

Stage 3

HO NH2 HO N C CH3

paracetamol

(a) In Stage 1, phenol is nitrated using dilute nitric acid.

The nitration of benzene requires concentrated nitric acid at 55°C with a catalyst

of concentrated sulfuric acid.

Both these reactions are electrophilic substitution.

(i) Explain why phenol can be nitrated using milder conditions than benzene.

(2)

(ii) A mixture of 2-nitrophenol and 4-nitrophenol is produced in Stage 1.

They are separated by steam distillation.

The boiling temperature of 2-nitrophenol is 215°C and that of

4-nitrophenol is 279°C.

14 Explain, in terms of intermolecular forces, why 4-nitrophenol has a higher

boiling temperature than 2-nitrophenol.*P67806A01436*

You may include a diagram in your answer.

(2)

(b) State the type of reagent needed to convert 4-nitrophenol into 4-aminophenol in

Stage 2.

(1)

• place 1.0 g of 4-aminophenol in a conical flask. Add 9 cm3 of distilled water

and stir the mixture

• add 1 cm3 of ethanoic anhydride to the flask and shake the mixture until a

precipitate of impure paracetamol forms

• remove the paracetamol by filtration under reduced pressure 15

• recrystallise the paracetamol using water as the solvent*P67806A01536*

• determine the melting temperature of the pure, dry paracetamol.

(i) Draw a labelled diagram of the apparatus used for filtration under

reduced pressure.

(3)

(ii) Describe the recrystallisation process to obtain a pure, dry sample

of paracetamol.

(5)

… *P67806A01636*

(iii) The melting temperature of pure paracetamol is 170°C.

Describe what happens to the melting temperature if the paracetamol is

not pure.

(2)

(d) The percentage by mass of paracetamol in three brands of paracetamol tablets

are shown in the table.

Percentage by mass of 17

Brand of tablet*P67806A01736*

paracetamol

P 92.1

Q 93.6

R 99.7

The amount of paracetamol in a tablet can be determined using a titration with

cerium(IV) ions.

The tablets are crushed and then hydrolysed in acid to form 4-aminophenol.

H O

HO N C CH3 + H2O HO NH2 + CH3COOH

4-aminophenol is oxidised by cerium(IV) ions.

HO NH + 2Ce4+ O NH + 2Ce3+ + 2H+

A tablet from one of the three brands of paracetamol was analysed following the

outline procedure.

• one of the tablets was crushed and 0.500g of the powder was added to

dilute sulfuric acid

• the mixture was heated under reflux until the hydrolysis was complete

• the solution was made up to 100.0 cm3 in a volumetric flask

• 25.0 cm3 portions of the solution were titrated against 0.100 mol dm−3 Ce4+

using ferroin as indicator

Result

The mean titre was 16.50 cm3.

(i) Give the molecular formula of paracetamol.

(1)

(ii) Determine, by calculation, which brand of tablet was analysed.

(5)

*P67806A01836*

(Total for Question 6 = 21 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

6(a)(i) An explanation that makes reference to the following (2)

points:

• the electron density of the (benzene) ring is greater in

phenol (than in benzene) (1)

• because the lone pair (of electrons) on oxygen Allow lone pair (of electrons) on oxygen feeds into

and / donates into / interacts with the delocalised

overlaps with the pi cloud / delocalised electrons / electrons / system

delocalised system (1) Ignore electron pushing effect of OH

Number

6(a)(ii) An explanation that makes reference to the following (2)

points:

• they both form hydrogen bonds (1) Allow M1 and M2 shown in diagrams

Ignore reference to other specific types of

intermolecular forces

• in 4-nitrophenol the hydrogen bonds join molecules in Allow 4-nitrophenol forms stronger

a straight chain / at both ends / at opposite ends (of the intermolecular hydrogen bonds / forces /

molecule so are stronger) interactions

2-nitrophenol forms intramolecular hydrogen bonds / Allow in 2-nitrophenol the hydrogen bonds join 2

forces / interactions (so fewer intermolecular hydrogen (1) molecules together / form a dimer (so there are

bonds) fewer / weaker hydrogen bonds)

Allow in 2-nitrophenol the hydrogen bonds are on

the same side (of the molecule)

Number

6(b) (1)

• reducing agent / reductant Ignore tin and concentrated hydrochloric acid

Do not award any other named reducing agent

Number

6(c)(i) Example of diagram (3)

• side-arm flask with label ‘to pump’ / drawing of pump (1) Ignore just ‘suction’

• (Buchner) funnel with perforations and bung around neck Allow funnel joined to flask with ‘Quickfit’ joint /

of funnel (1) no gap

• flat filter paper (over perforations) (1) Do not award fluted filter paper / filter paper that

extends up the sides of the funnel

Number

6(c)(ii) A description that makes reference to the following (5)

points:

• dissolve crystals in the minimum (amount / volume) (1) Allow add / put for dissolve

Do not award wash for dissolve

• of hot water / solvent

(1) Penalise use of incorrect solvent once only

• filter hot and allow to cool

(1) Allow M3 if hot is omitted and is mentioned in M2

• filter and wash with a small amount of (cold) solvent Do not award filter to remove soluble impurities

(1)

• dry crystals between filter papers / in a desiccator / in a Stand alone mark

warm oven (1) Allow other suitable methods of drying

Do not award reference to crystals mixed with a

drying agent

Number

6(c)(iii) An answer that makes reference to the following points: (2)

• melting temperature is lower (1)

• it melts over a range of temperatures Allow a specified range of temperatures

or (1)

the melting temperature is not sharp

Number

6(d)(i) Allow the symbols, with subscripts, in any order (1)

• C8H9NO2 e.g. C8H9O2N

Allow large numbers but not superscripts

Ignore any other formulae as working

Number

6(d)(ii) Example of calculation (5)

amount Ce4+ used = 16.5 x 0.100

• calculation of amount of Ce4+ (1) 1000

= 0.00165 / 1.65 x 10−3 (mol)

3 (1) amount paracetamol in 25.0 cm3 = 0.00165

• calculation of amount of paracetamol in 25.0 cm

= 0.000825 / 8.25 x 10−4 (mol)

TE on M1

3 (1) amount paracetamol in 100.0 cm3

• calculation of amount of paracetamol in 100.0 cm

= 4 x 0.000825

= 0.00330 / 3.30 x 10−3 (mol)

TE on M2

• calculation of molar mass of paracetamol (1) molar mass of paracetamol, C8H9NO2

and = (8 x 12) + (9 x 1) + 14 + (2 x 16) = 151

mass of paracetamol in 1 tablet and

mass of paracetamol in 1 tablet

= 0.00330 x 151 = 0.4983 g

TE on M3 and (d)(i)

• calculation of percentage of paracetamol in 1 tablet (1) percentage of paracetamol

and = 0.4983 x 100 = 99.66 (%)

conclusion 0.500

and

the tablet was from Brand R

TE on M4 provided M4 < 0.500 g

Ignore SF except 1 SF

(Total for Question 6 = 21 marks)

How to answer it

Preparation and Analysis of Paracetamol

📚 What this question tests

This multi-step synoptic question assesses core A-Level Organic Chemistry topics: activating groups in electrophilic substitution (phenol vs. benzene), intermolecular forces (inter- vs. intra-molecular hydrogen bonding), practical techniques (filtration under reduced pressure, recrystallization, and melting point determination), organic functional group reduction, molecular formula derivation, and complex multi-stage titration stoichiometry.

Question 6 (a)(i)

Nitration of Phenol vs. Benzene

Explain why phenol can be nitrated using milder conditions than benzene. (2 marks)

✅ Correct Answer

The electron density of the benzene ring is greater in phenol than in benzene.
The lone pair of electrons on the oxygen atom overlaps with the pi cloud / delocalised system of the ring.

💡 Key Knowledge

The -OH group is strongly electron-donating due to the p-type lone pair on oxygen interacting directly with the delocalised pi electrons of the benzene ring. This activates the ring, making it far more nucleophilic and susceptible to electrophilic attack.

Mark breakdown: 1 mark for stating phenol has higher electron density; 1 mark for explaining that the oxygen lone pair delocalises into the pi system.

Question 6 (a)(ii)

Boiling Temperatures of Nitrophenol Isomers

Explain, in terms of intermolecular forces, why 4-nitrophenol has a higher boiling temperature than 2-nitrophenol. (2 marks)

✅ Correct Answer

Both molecules form hydrogen bonds.
4-nitrophenol: Hydrogen bonds join molecules in a straight chain (intermolecular), leading to stronger overall intermolecular forces.
2-nitrophenol: Forms intramolecular hydrogen bonds (within the same molecule), meaning fewer/weaker hydrogen bonds are available between molecules.

❌ Common Errors

Students frequently confuse intra-molecular (within the molecule) and inter-molecular (between molecules) hydrogen bonding, or incorrectly state that 2-nitrophenol does not form hydrogen bonds at all.

Mark breakdown: 1 mark for noting both form hydrogen bonds; 1 mark for linking 4-nitrophenol to intermolecular/chain-forming H-bonds and 2-nitrophenol to intramolecular H-bonds.

Question 6 (b)

Reduction of Nitro Groups

State the type of reagent needed to convert 4-nitrophenol into 4-aminophenol in Stage 2. (1 mark)

✅ Correct Answer

Reducing agent / reductant (Accept: Tin and concentrated hydrochloric acid).

🧠 Exam Technique

Keep terminology precise. Do not over-elaborate unless requested; stating "reducing agent" directly matches the mark scheme.

Mark breakdown: 1 mark for "reducing agent" or "reductant".

Question 6 (c)(i)

Buchner Filtration Apparatus

Draw a labelled diagram of the apparatus used for filtration under reduced pressure. (3 marks)

✅ Correct Answer

Side-arm filter flask clearly connected to a vacuum pump / source with the label "to pump".
Buchner funnel fitted securely into the neck using a bung or rubber adapter with visible perforations inside the base.
Flat filter paper resting completely flat covering the perforations inside the Buchner funnel.

❌ Common Errors

Drawing fluted filter paper or paper that creeps up the sides of the funnel will lose the final mark, as air would bypass the precipitate.

Mark breakdown: 1 mark for side-arm flask + pump label; 1 mark for Buchner funnel with perforations & bung; 1 mark for flat filter paper.

Question 6 (c)(ii)

Recrystallisation Procedure

Describe the recrystallisation process to obtain a pure, dry sample of paracetamol. (5 marks)

✅ Correct Answer

Dissolve impure crystals in a minimum volume of hot solvent/water.
Filter the hot solution to remove insoluble impurities, then allow it to cool so crystals form.
Filter the cooled mixture (under reduced pressure) and wash with a small amount of cold solvent.
Dry the pure crystals between filter papers, in a desiccator, or in a warm oven.

🧠 Exam Technique

Keywords are vital here: always specify "minimum volume of hot solvent", "hot filtration", and "cold solvent wash" to secure all 5 marks.

Mark breakdown: 1 mark per bullet point (Minimum hot solvent, hot filtration, cooling, cold wash, drying).

Question 6 (c)(iii)

Effect of Impurities on Melting Temperature

Describe what happens to the melting temperature if the paracetamol is not pure. (2 marks)

✅ Correct Answer

The melting temperature is lower than the literature value.
The sample melts over a range of temperatures (not sharp).

💡 Key Knowledge

Impurities disrupt the regular crystal lattice structure, requiring less thermal energy to break intermolecular forces, which both depresses and broadens the melting range.

Mark breakdown: 1 mark for lower melting temp; 1 mark for melting over a range.

Question 6 (d)(i)

Molecular Formula Determination

Give the molecular formula of paracetamol. (1 mark)

✅ Correct Answer

C₈H₉NO₂ (Symbols may be in any order, e.g., C₈H₉O₂N ).

❌ Common Errors

Do not use superscript formatting or spaces inside the chemical formula.

Mark breakdown: 1 mark for correct molecular formula.

Question 6 (d)(ii)

Titration Calculation & Tablet Identification

Determine, by calculation, which brand of tablet was analysed. (5 marks)

📐 Step-by-Step Calculation

Amount of Ce⁴⁺ used in titration:
(16.50 × 0.100) / 1000 = 0.00165 mol (or 1.65 × 10⁻³ mol)
Amount of paracetamol in 25.0 cm³ portion:
From equation, ratio is 1 paracetamol : 2 Ce⁴⁺
0.00165 / 2 = 0.000825 mol
Amount of paracetamol in total 100.0 cm³ flask:
0.000825 × (100.0 / 25.0) = 0.00330 mol
Molar mass and mass in 1 tablet:
M(C₈H₉NO₂) = (8×12) + (9×1) + 14 + (2×16) = 151 g mol⁻¹
Mass = 0.00330 mol × 151 g mol⁻¹ = 0.4983 g
Percentage by mass & conclusion:
(0.4983 / 0.500) × 100 = 99.66%
Conclusion: Tablet was from Brand R (99.7%).

Mark breakdown: 1 mark for moles of Ce⁴⁺; 1 mark for moles in 25 cm³; 1 mark for moles in 100 cm³; 1 mark for mass of paracetamol in tablet; 1 mark for percentage calculation and correct identification of Brand R.

Topics

Organic Chemistry · Physical Chemistry · Core Practicals · Topic 18: Organic Chemistry III · Topic 17: Organic Chemistry II · Topic 6: Organic Chemistry I · Topic 5: Formulae, Equations and Amounts of Substance · Topic 2: Bonding and Structure · Core Practical 15: Analyse organic and inorganic unknowns · Core Practical 7: Identify unknown organic liquids and inorganic solids

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