Edexcel A-Level Chemistry Paper 1, November 2020: Question 2

✅ Correct Answer

A ( CH₃COOH > CH₂ClCOOH > HCl )

Awarded 1 mark for selecting A.

💡 Key Knowledge

• Strong acid (HCl): Completely dissociates, producing the highest concentration of H⁺ ions and therefore the lowest pH.
• Inductive Effect: Chlorine is electronegative and pulls electron density away via inductive effects, stabilizing the carboxylate anion. This makes chloroethanoic acid ( CH₂ClCOOH ) stronger than ethanoic acid ( CH₃COOH ).
• Stronger acids have lower pH values. Therefore, decreasing pH order means going from weakest acid to strongest acid.

📐 Step-by-Step Calculation

1. Calculate [H⁺]:
   [H⁺] = 10⁻ᵖᴴ = 10⁻²·²⁰ = 6.3096 × 10⁻³ mol dm⁻³ (1 mark)
2. Set up and use the Kₐ expression:
   Assuming [H⁺] ≈ [HCOO⁻] and [HCOOH]ₑᵾᵢₗᵢᵦᵣᵢᵤₘ ≈ initial concentration ( 0.240 mol dm⁻³ ):
   Kₐ = [H⁺]² / [HA] = (6.3096 × 10⁻³)² / 0.240 = 1.6588 × 10⁻⁴ mol dm⁻³ (1 mark)
3. Calculate pKₐ:
   pKₐ = -log(Kₐ) = -log(1.6588 × 10⁻⁴) = 3.78 (or 3.7802) (1 mark)

❌ Common Errors & Traps

• Concentration approximation trap: Forgetting that weak acids partially dissociate, but for standard calculations at A-Level, initial concentration equals equilibrium undissociated concentration.
• Logarithm mistakes: Forgetting the negative sign when converting Kₐ to pKₐ.
• Note: Correct final answer with no working automatically scores full marks (3/3), but showing working protects against arithmetic errors.

✅ Correct Answer

B ( NaOH(aq) and excess CH₃COOH(aq) )

Awarded 1 mark for selecting B.

🧠 Exam Technique & Knowledge

• A buffer solution requires a weak acid in excess mixed with a salt of that weak acid (or generated in situ by reacting the weak acid with a limited amount of strong alkali like NaOH).
• Option B reacts NaOH with excess CH₃COOH to produce CH₃COONa while leaving unreacted CH₃COOH , creating an acidic buffer (pH below 7).
• Options A and C involve hydrochloric acid ( HCl ), which creates acidic solutions or buffers not based on weak organic systems in this context, while D involves excess alkali which yields a pH above 7.

✅ Correct Answer

D (none of these three indicators)

Awarded 1 mark for selecting D.

💡 Key Knowledge: Indicator Selection

• Weak Acid – Weak Base Titrations: Titrating ammonia ( NH₃(aq) , a weak base) with ethanoic acid ( CH₃COOH(aq) , a weak acid) results in a titration curve with no sharp vertical pH inflection at the equivalence point.
• Standard indicators (methyl orange, bromothymol blue, phenolphthalein) change color over specific, narrow pH ranges that require a sharp pH jump. Because the pH change during a weak-weak titration is gradual, no standard single indicator changes color sharply at the exact equivalence point.