Edexcel A-Level Chemistry Paper 3, June 2025: Question 9

21 marks · Hard difficulty · Synoptic Questions

Complete the mechanism for the sulfonation of 1,4-dimethylbenzene, describe recrystallisation and dilution techniques, and perform back-titration calculations with error evaluation.

Practise this question

Question

Question 9 spans multiple parts across several pages, totaling 21 marks. Part (a) asks to complete the electrophilic substitution mechanism of 1,4-dimethylbenzene with sulfur trioxide to form 2,5-dimethylbenzenesulfonic acid. Part (b) outlines a recrystallisation protocol, asking (i) how it removes soluble impurities (3 marks) and (ii) how to modify it to remove insoluble impurities (2 marks). Part (c) describes a back-titration experiment to find water of crystallisation n: (i) explain how to make the diluted solution in a 100 cm³ volumetric flask (4 marks) and (ii) calculate the relative formula mass of the hydrated acid and deduce n (5 marks). Part (d) shows an illustration of a student pouring acid through a funnel into a high-standing burette above head height: (i) suggest one change to reduce risk (1 mark), and (ii) explain the effect on the calculated value of n if acid drips from the funnel into the burette during titration (3 marks).
Question text

9 This question is about the compound 2,5-dimethylbenzenesulfonic acid, which has

the structure shown.

O

HO S O

It is used to analyse levels of cholesterol in blood plasma. It can be synthesised by the

reaction of 1,4-dimethylbenzene with concentrated sulfuric acid.

The electrophile in this reaction is sulfur trioxide.

(a) Complete a possible mechanism for this reaction by adding curly arrows, and any

relevant dipoles, charges and lone pairs.

(3)

O

S

O O

structure of intermediate

O

HO S O

(b) The 2,5-dimethylbenzenesulfonic acid formed can be purified by recrystallisation

using hot water as a solvent.

Procedure

Step 1 Dissolve impure 2,5-dimethylbenzenesulfonic acid in the minimum

28 volume of hot water.

Step2 Allow the solution to cool until 2,5-dimethylbenzenesulfonic acid*P77834A02836*

has crystallised.

Step 3 Separate the 2,5-dimethylbenzenesulfonic acid by filtering under

reduced pressure.

Step 4 Rinse the 2,5-dimethylbenzenesulfonic acid using a small amount of

ice-cold water.

Step 5 Dry the 2,5-dimethylbenzenesulfonic acid between two pieces of

filter paper.

(i) Explain how recrystallisation reduces the amount of soluble impurities.

(3)

(ii) A student carrying out this procedure noticed some insoluble impurities in

their sample.

Describe how the procedure should be amended to remove these

insoluble impurities.

(2)

(c) The hydrated form of the acid has the formula C6H3(CH3)2SO3H•nH2O.

An experiment was carried out to determine the number of moles of water of

crystallisation, n, in this formula.

The method involves adding excess sodium hydroxide to a known mass of the 29

hydrated 2,5-dimethylbenzenesulfonic acid.*P77834A02936*

The amount of unreacted sodium hydroxide is then determined by titration.

Procedure

Step 1 10.0 cm3 of 1.00 mol dm−3 NaOH(aq) was added to 1.60 g of hydrated

2,5-dimethylbenzenesulfonic acid in a small conical flask.

Step 2 The solution formed was diluted to give a volume of exactly 100 cm3.

Step 3 20.0 cm3 samples of the diluted solution required a mean titre of

22.40 cm3 of 0.02500 mol dm−3 HCl(aq) to neutralise the unreacted

sodium hydroxide present in the sample.

(i) Describe how to make the diluted solution, in Step 2.

(4)

(ii) Calculate the relative formula mass of hydrated*P77834A03036*

2,5-dimethylbenzenesulfonic acid and hence deduce the value of n.

(5)

(d) A student carrying out this experiment used a funnel as shown to try to ensure

the acid was added safely to the burette during the titration.

(i) Give one change to this technique that would reduce the risk to this student.

(1)

(ii) The student forgot to remove the funnel from the burette and during the

titration five drops of acid from the funnel dripped into the burette.

Explain what effect, if any, this error will have on the final value for n.

(3)

… *P77834A03136*

(Total for Question 9 = 21 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 9 giving criteria for all parts. 9(a) awards 3 marks for 6 marking points including curly arrow from benzene ring to S, dipoles, intermediate arenium ion structure with delocalised positive charge, negative charge on oxygen, and deprotonation to reform aromaticity. 9(b)(i) awards 3 marks for solubility in hot/cold solvent and remaining in solution upon cooling. 9(b)(ii) awards 2 marks for hot filtration before cooling. 9(c)(i) awards 4 marks for volumetric flask technique including rinsings and inversion. 9(c)(ii) provides step-by-step calculation: moles of HCl = 5.60 x 10^-4, unreacted NaOH = 2.80 x 10^-3, reacted acid = 7.20 x 10^-3, Mr = 222.2, giving n = 2. 9(d)(i) awards 1 mark for lowering the burette below eye level. 9(d)(ii) awards 3 marks for explaining that drops reduce the titre, meaning a smaller calculated excess NaOH, greater calculated reacting acid, smaller Mr, and thus a smaller calculated value of n.

Question

Answer Additional Guidance Mark

Number

9(a) An answer that makes reference to the following points: (3)

• arrow from inside the hexagon to sulfur atom

• partial charges on S and O

• basic structure of intermediate, including both

methyl groups, and broken circle covering at least

three carbons, facing the tetrahedral carbon, and

containing (some part of) positive charge

• lone pair

and

negative charge on O of intermediate

In point 2 allow δ− on all O atoms

In point 5 allow arrow from lone pair on O to H+,

• arrow from (lone pair on) oxygen to H on ring +

or hydrogen on H2O, H3O , or H2SO4

• return of C−H bond pair towards/ inside (broken)

ring 6 points scores 3 marks

4 or 5 points scores 2 marks

2 or 3 points scores 1 mark

0 or 1 point scores 0 marks

Question

Answer Additional Guidance Mark

Number

9(b)(i) An explanation that makes reference to the following points: (3)

• both 2,5-dimethylbenzenesulfonic acid and (soluble)

impurities dissolve in hot water / hot solvent (1)

• impurities are soluble in cold solvent Allow impurities are present in small amounts

or (and so do not crystallise in cold solvent)

2,5-dimethylbenzenesulfonic acid is (much) less soluble / (1) Do not award the ice-cold water (in step 4)

insoluble in cold solvent dissolves the soluble impurities

• so impurities remain in solution (as the solvent cools) / (1) Allow the ice-cold water washes off any

impurities remain in the filtrate surface impurities

and

2,5-dimethylbenzenesulfonic acid recrystallises / solid

remains (on filter paper)

Question

Answer Additional Guidance Mark

Number

9(b)(ii) A description that makes reference to the following points: (2)

• filter when still hot / filter funnel and / or paper are heated (1)

• (filtration) carried out after step 1 / before step 2 (to (1) Allow redissolve in a different solvent

remove insoluble impurities)

If no other credit awarded, allow 1 mark for

decant (off from the impurities) whilst still

hot

Question

Answer Additional Guidance Mark

Number

9(c)(i) A description that makes reference to the following points: (4)

• (add the solution formed) to a 100 cm3 volumetric flask (1) Ignore starting with just the acid and omitting

the alkali

• (rinse remaining solution from conical flask) with (1)

distilled / deionised water and add washings to volumetric

flask

• add distilled / deionised water (1)

• make up to the line / mark (1)

and

• (add stopper) then invert / mix (to ensure solution is Do not award shake

homogenous)

Question

Answer Additional Guidance Mark

Number

9(c)(ii) Example of calculation (5)

• calculation of moles of HCl(aq) (1) (22.4/1000) × 0.0250 = 5.6 × 10–4 (mol)

(1) 5.6 × 10–4 × 5 = 2.8 × 10–3 (mol)

• calculation of moles of NaOH in in small conical flask

(1) (10/1000) – 2.8 × 10–3 = 7.2 × 10–3

• calculation of moles of 2,5-dimethylbenzenesulfonic acid

(1) 1.60 / 7.2 × 10–3 = 222.2 (g mol–1)

• calculation of Mr of 2,5-dimethylbenzenesulfonic acid

• deduction of value for n (must be an integer) (1) 222.2 – 186.1 = 36.1 so n = 2

Allow TE throughout

Correct answer with no working scores M5

only

If you see 222.2 for the Mr this will score

M1-M4 as the other steps will have been

carried out correctly.

Do not award negative value of n

Question

Answer Additional Guidance Mark

Number

9(d)(i) An answer that makes reference to the following point: (1)

• lower burette so pouring in acid below eye-level / Allow alternative methods of height reduction

place stand in sink / on (stable) stool / on floor Ignore just an observation that the top of the burette is

too high

Ignore just lower the burette / stand

Do not award student raises themselves e.g. standing

on a stool

Question

Answer Additional Guidance Mark

Number

9(d)(ii) An explanation that makes reference to the following (3)

points:

• reduce the value of the titre (1)

• smaller (calculated) value of excess NaOH / greater (1) M3 dependent on M2

(calculated) moles of 2,5-dimethylsulfonic acid that Allow smaller Mr but negligible effect on

reacted (calculated value) of n

If M1 is answered as ‘increase the value of the

• so smaller Mr, hence smaller (calculated value of) n (1) titre’ then allow reverse arguments for M2 and

M3

(Total for Question 9 = 21 marks)

TOTAL FOR PAPER = 120 MARKS

How to answer it

Synthesis, Purification & Analysis of 2,5-Dimethylbenzenesulfonic Acid

WHAT THIS QUESTION TESTS

This comprehensive 21-mark practical and physical-organic question tests:

  • Electrophilic Aromatic Substitution: Drawing the curved arrow mechanism with neutral electrophile sulfur trioxide (SO₃), including dipole generation, horseshoe intermediate carbocation, and proton loss.
  • Organic Practical Skills: Principles of recrystallisation, removal of soluble vs insoluble impurities (hot filtration), and volumetric flask standard solution preparation.
  • Quantitative Back Titration: Multi-stage stoichiometry to find molar mass and water of crystallisation ( n ).
  • Apparatus & Error Propagation: Laboratory safety during burette filling and tracing systematic error propagation through titration calculation steps.

Part (a): Mechanism of Electrophilic Substitution

Electrophilic attack of sulfur trioxide on 1,4-dimethylbenzene (3 Marks)

✅ Mechanism Checklist (6 Key Scoring Points)

  1. Initial attack: Curly arrow originating from inside the benzene delocalised pi-system pointing directly to the sulfur atom of SO₃.
  2. Dipoles & SO₃ cleavage: Partial charges on SO₃ (δ+ on S, δ- on O atoms) AND a curly arrow from the S=O double bond onto the oxygen atom.
  3. Intermediate structure: Hexagonal ring with both methyl groups, tetrahedral carbon with bonded -H and -SO₃⁻, and an open horseshoe carbocation spanning at least 3 carbon atoms facing the sp³ carbon, containing a positive charge (+).
  4. Intermediate charges: A full negative charge ( - ) and lone pair on the single-bonded oxygen of the sulfonate group.
  5. Proton transfer: Curly arrow from the lone pair on the sulfonate oxygen atom (or a base/water molecule) to the H atom attached to the tetrahedral carbon.
  6. Aromatic regeneration: Curly arrow from the C-H bond pointing back into the broken ring carbocation.

🧠 Exam Technique & Mark Scheme Conversion

This 3-mark question uses an aggregate points system:

  • 6 points: 3 marks
  • 4 or 5 points: 2 marks
  • 2 or 3 points: 1 mark
  • 0 or 1 point: 0 marks

Draw with precision: Make sure your horseshoe does not extend past the meta carbons (it must span 5 carbons total, open at the tetrahedral carbon). Ensure the '+' charge is placed strictly inside the horseshoe.

❌ Common Errors in this Mechanism

  • Starting the first arrow from a localized carbon atom instead of the inside of the delocalised pi-electron ring.
  • Forgetting to show the formal negative charge on the oxygen atom in the -SO₃⁻ intermediate.
  • Drawing the arrow from the hydrogen atom rather than from the C-H bond back into the ring.
Mark Scheme: Maximum 3 marks awarded via aggregate score of the 6 detailed points.

Part (b): Recrystallisation & Purification

Principles of solubility separation and hot filtration (5 Marks Total)

💡 (b)(i) How Recrystallisation Removes Soluble Impurities (3 Marks)

  • Mark 1: Both the 2,5-dimethylbenzenesulfonic acid and the soluble impurities completely dissolve in the hot water / solvent.
  • Mark 2: Soluble impurities are soluble in cold water / present in small quantities, whereas 2,5-dimethylbenzenesulfonic acid is much less soluble / insoluble in cold water.
  • Mark 3: As the solution cools, the acid crystallises out (remains on filter paper), while the soluble impurities remain dissolved in the solution / filtrate (and are washed away).

✅ (b)(ii) Removing Insoluble Impurities (2 Marks)

  • Mark 1 (Technique): Carry out hot filtration (filter while the solution is still hot, using a pre-heated funnel / fluted filter paper).
  • Mark 2 (Placement): Perform this filtration after Step 1 and before Step 2 (i.e. before allowing the solution to cool and crystallise).

❌ Common Misconceptions in Recrystallisation

  • Confusing the steps: Filtering after cooling would trap both insoluble impurities AND the desired crystallised product together on the filter paper!
  • Cold wash purpose: Step 4 (rinsing with ice-cold water) washes off soluble impurities adhering to the surface of the crystals; it does not dissolve the bulk impurities.
Mark Scheme: (b)(i) 3 marks for dissolve hot → cold solubility difference → separation into solid vs filtrate. (b)(ii) 1 mark for hot filtration, 1 mark for correct stage.

Part (c)(i): Making the Diluted Standard Solution

Practical method for Step 2 dilution (4 Marks)

✅ Standard Solution Preparation Steps (4 Marks)

  1. Transfer: Transfer the reaction mixture from the conical flask into a 100 cm³ volumetric flask.
  2. Rinsing: Rinse the conical flask with distilled (deionised) water and add the washings into the volumetric flask.
  3. Make up to volume: Add distilled water until the bottom of the meniscus touches the graduation mark at eye level.
  4. Homogenise: Insert the stopper and invert / invert repeatedly to ensure the solution is thoroughly mixed.

❌ Examiner Trap

"Shaking" vs "Inverting": The mark scheme specifically states: "Do not award shake". Always state: "stopper and invert to mix" to guarantee Mark 4.

Mark Scheme: 1 mark for each of the 4 clear procedural steps.

Part (c)(ii): Back Titration Calculation

Determining Molar Mass and Water of Crystallisation, n (5 Marks)

📐 Step-by-Step Calculation

Step 1: Calculate moles of HCl used in titration (Mark 1)
Moles of HCl = (volume / 1000) × concentration
Moles of HCl = (22.40 / 1000) × 0.02500 = 5.60 × 10⁻⁴ mol
Step 2: Moles of excess NaOH in 20.0 cm³ sample and in total 100 cm³ (Mark 2)
NaOH + HCl → NaCl + H₂O (1:1 ratio)
Moles of excess NaOH in 20.0 cm³ = 5.60 × 10⁻⁴ mol
Scaling factor = 100.0 cm³ / 20.0 cm³ = 5
Total excess NaOH in 100 cm³ flask = 5.60 × 10⁻⁴ × 5 = 2.80 × 10⁻³ mol
Step 3: Moles of NaOH that reacted with sulfonic acid (Mark 3)
Initial NaOH added = (10.0 / 1000) × 1.00 = 1.00 × 10⁻² mol
Moles reacted = Initial NaOH - Excess NaOH
Moles reacted = 1.00 × 10⁻² - 2.80 × 10⁻³ = 7.20 × 10⁻³ mol
Since 2,5-dimethylbenzenesulfonic acid is monoprotic (1:1 ratio):
Moles of hydrated acid = 7.20 × 10⁻³ mol
Step 4: Relative formula mass (Mᵣ) of hydrated acid (Mark 4)
Mass of sample = 1.60 g
Mᵣ = mass / moles = 1.60 / (7.20 × 10⁻³) = 222.2 g mol⁻¹
Step 5: Deduce the integer value of n (Mark 5)
Formula of anhydrous acid = C₆H₃(CH₃)₂SO₃H = C₈H₁₀SO₃
Mᵣ(anhydrous) = (8 × 12.0) + (10 × 1.0) + 32.1 + (3 × 16.0)
Mᵣ(anhydrous) = 96.0 + 10.0 + 32.1 + 48.0 = 186.1 g mol⁻¹
Mass of water of crystallisation = 222.2 - 186.1 = 36.1 g mol⁻¹
n = 36.1 / 18.0 = 2.006 → n = 2

❌ Calculation Pitfalls

  • Forgetting to scale up: Failing to multiply the titration moles by 5 (100/20) is the most frequent blunder.
  • Integer requirement: The question asks for the value of n. Writing 2.01 without rounding to the integer 2 forfeits Mark 5.
Mark Scheme: 1 mark per correct calculation stage. Transfer of error (TE) allowed throughout. Final answer must be an integer (n = 2).

Part (d): Safety & Error Analysis

Apparatus risk reduction and titration error propagation (4 Marks Total)

🧠 (d)(i) Safety Modification (1 Mark)

Correction: Lower the burette so that the top is below eye level when pouring acid (e.g. place the stand in a sink or safely on the floor / lower clamp).

Examiner Warning: Do not suggest the student stands on a stool or chair (this introduces a fall hazard!). Do not simply write "lower the burette" without explaining that pouring must occur below eye level.

💡 (d)(ii) Effect of Acid Dripping into Burette (3 Marks)

Follow the chain of reasoning carefully:

  1. Effect on Titre (Mark 1): Dripping extra acid into the burette during titration increases liquid level, meaning the delivered volume appears smaller → reduces the value of the titre.
  2. Effect on Acid Reacted (Mark 2): A smaller titre means a lower calculated value for unreacted excess NaOH. Consequently, the calculated moles of reacted sulfonic acid is greater.
  3. Effect on n (Mark 3): A greater calculated number of moles of acid means calculated Mᵣ is smaller (Mᵣ = mass / moles). A smaller Mᵣ means less mass is attributed to water, so the calculated value of n is smaller.
Mark Scheme: (d)(i) 1 mark for pouring below eye level / on floor / in sink. (d)(ii) 3 marks for linking: lower titre → greater reacted acid moles → smaller Mᵣ and smaller n.

Topics

Organic Chemistry · Physical Chemistry · Core Practicals · Topic 18: Organic Chemistry III · Topic 5: Formulae, Equations and Amounts of Substance · Core Practical 2: Preparation of a standard solution from a solid acid · Core Practical 3: Find the concentration of a solution of hydrochloric acid

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.