AQA A-Level Chemistry Paper 3, June 2018: Question 32
1 mark · Medium difficulty · Multiple Choice
Identify which indicator is suitable for the titration of the weak base methylamine with hydrochloric acid.
Practise this questionQuestion
Question text
32 Which indicator should be used in a titration to find the concentration of a solution of
methylamine using 0.010 mol dm–3 hydrochloric acid?
[1 mark]
A Thymol blue (pH range 1.2–2.8)
B Bromophenol blue (pH range 3.0–4.6)
C Phenol red (pH range 6.8–8.4)
D Phenolphthalein (pH range 8.3–10.0)
Mark scheme
Show the mark scheme
32 B
How to answer it
Indicator Choice for Methylamine Titration
This question evaluates your understanding of acid-base titration curves and indicator selection:
- Classifying substances as strong/weak acids and strong/weak bases (HCl is a strong monoprotic acid; methylamine, CH₃NH₂, is a weak organic base).
- Predicting the vertical pH range and equivalence point for a strong acid – weak base titration.
- Matching the pH transition range of an indicator to the steep, vertical section of a pH curve.
Question 32
Multiple Choice (1 Mark)
✅ Correct Answer
B: Bromophenol blue (pH range 3.0–4.6)
💡 Key Knowledge
- Acid/Base Strength:
• HCl = strong acid (fully dissociated)
• CH₃NH₂ = weak base (partially ionised) - Salt Formed: Methylammonium chloride (CH₃NH₃⁺Cl⁻). The cation hydrolyses water:
CH₃NH₃⁺ + H₂O ⇌ CH₃NH₂ + H₃O⁺
This makes the solution acidic at equivalence (pH typically around 4–6). - Vertical Section: For a weak base – strong acid titration, the vertical drop occurs in the acidic region (roughly pH 3 to 7).
📐 Step-by-Step Decision Process
- Identify Titration Type: Titration of a weak base with a strong acid.
- Locate Equivalence Point & Vertical Region: Because the conjugate acid of methylamine is weakly acidic, the vertical section lies below pH 7, generally between pH 3.0 and pH 6.5.
- Evaluate Indicators:
- A: Thymol blue (1.2–2.8) — Too low; changes color only after adding a large excess of HCl.
- B: Bromophenol blue (3.0–4.6) — Ideal; its range lies entirely within the steep vertical drop.
- C: Phenol red (6.8–8.4) — Lies near neutral/alkaline, before the sharp equivalence drop occurs.
- D: Phenolphthalein (8.3–10.0) — Used for weak acid – strong base titrations; will change color far too early.
❌ Common Errors & Examiner Traps
- Assuming equivalence is always pH 7: Many students think neutralisation always results in pH 7 and incorrectly choose Phenol red (6.8–8.4).
- Confusing the standard indicators: Phenolphthalein (8.3–10.0) is the default indicator for weak acid – strong base titrations (e.g. ethanoic acid with NaOH). Picking it here shows confusion between weak acid/strong base and weak base/strong acid.
- Over-compensating towards extreme acidity: Thymol blue (1.2–2.8) changes colour in very strongly acidic conditions, well past the equivalence point of a 0.010 mol dm⁻³ titration.
🧠 Exam Technique & Revision Rule of Thumb
Always remember the golden rule for indicator choice: The indicator's pH range must fall completely within the vertical/steep section of the pH titration curve.
- Strong Acid + Strong Base: Vertical section ~3 to 11 → Methyl orange or Phenolphthalein.
- Strong Acid + Weak Base: Vertical section ~3 to 7 (acidic) → Methyl orange / Bromophenol blue (pH 3–5 range).
- Weak Acid + Strong Base: Vertical section ~7 to 11 (basic) → Phenolphthalein (pH 8–10 range).
- Weak Acid + Weak Base: No steep vertical section → No indicator suitable (must use a pH meter).
Topics
Physical Chemistry · Organic Chemistry · 3.1.12 Acids and Bases · 3.3.11 Amines
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.