OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify a suitable indicator pH range for titrating remaining ethanoic acid in an equilibrium mixture with sodium hydroxide solution.

Practise this question

Question

Question 13 describes an equilibrium mixture of ethanoic acid and ethanol. A student titrates the remaining ethanoic acid with sodium hydroxide solution. A table lists four options for the pH range of an indicator: A (1.2 to 2.8), B (4.8 to 6.0), C (9.4 to 10.6), and D (12.0 to 13.4).

Mark scheme

Show the mark scheme Mark scheme for Question 13 showing the correct answer as 'C' with 1 mark allocated.

How to answer it

OCR A-Level Chemistry • Module 5: Physical Chemistry & Transition Elements

Choosing a Suitable Indicator for a Weak Acid–Strong Base Titration

What this question tests

This multiple-choice question assesses your ability to deduce the nature of an acid-base neutralization and select an appropriate chemical indicator based on the vertical section of a pH titration curve.

  • Identifying the acid-base type: weak acid (ethanoic acid, CH₃COOH) titrated with a strong base (sodium hydroxide, NaOH).
  • Understanding why the equivalence point of a weak acid–strong base titration lies in the alkaline region (pH > 7).
  • Matching the indicator's pH transition range with the steep vertical section of the titration curve.

Question 13 Breakdown

Equilibrium Mixture Analysis via Acid–Base Titration [1 Mark]

✅ Correct Answer

C (pH range: 9.4 – 10.6)

Mark Scheme Breakdown:
• Selecting C earns [1 mark].

💡 Key Knowledge

  • Reaction:
    CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)
  • Weak Acid vs Strong Base: The ethanoate ion (CH₃COO⁻) undergoes slight hydrolysis in water, producing OH⁻ ions:
    CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
  • Equivalence Point: Because of basic hydrolysis, the equivalence point occurs at pH > 7 (typically around pH 8.5 – 9.0).
  • Vertical Region: The vertical section of the pH curve for a weak acid–strong base titration typically lies in the range of approximately pH 7 to 11.

🧠 Exam Technique & Step-by-Step Selection

  1. Identify the reactants: Ethanoic acid is a weak organic acid; sodium hydroxide is a strong alkali. Ethanol and ethyl ethanoate present in the esterification equilibrium mixture do not react with NaOH under room-temperature titration conditions.
  2. Predict the vertical section: A weak acid–strong base curve starts around pH 3, buffers, and exhibits a sharp vertical climb entirely in the alkaline region (roughly pH 7.0 – 11.0).
  3. Rule for indicators: The end-point pH range of the indicator must fall completely (or almost completely) within the steep vertical portion of the curve.
  4. Evaluate the options:
    • A (1.2 – 2.8): Too acidic; changes color before the titration begins.
    • B (4.8 – 6.0): Falls in the buffer region of ethanoic acid; would change color long before the equivalence point.
    • C (9.4 – 10.6): Lies squarely within the vertical jump (similar to phenolphthalein or thymol blue). Correct!
    • D (12.0 – 13.4): Too high; reaches past the vertical section where the curve flattens out into excess NaOH.

❌ Common Errors & Pitfalls

  • Choosing an indicator based on pKa of the acid: Many students see the pKa of ethanoic acid (~4.76) and mistakenly choose B (4.8 – 6.0), forgetting that indicator colour change must coincide with the equivalence point, not the half-neutralisation point!
  • Confusing Strong Acid–Weak Base with Weak Acid–Strong Base: Strong acid–weak base titrations have a vertical section in the acidic region (pH 3 – 7, e.g., methyl orange). Students frequently invert these two scenarios.
  • Thinking indicator pH must include pH 7: An equivalence point of pH 7 only applies to strong acid–strong base titrations.

Topics

Module 5: Physical chemistry and transition elements · Practical Activity Groups · 5.1 Rates, equilibrium and pH · PAG 2: Acid-base titration

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.