Two students dissolve equal moles of HCl and CH₃COOH separately in water and measure the pH. Student A finds that the HCl solution has a lower pH than the CH₃COOH solution of equal concentration. Student B argues both should have the same pH since equal moles of acid were used. Who is correct, and what concept explains the difference?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
Student A is correct.
HCl is a strong acid — it ionises completely in water, producing a high concentration of H⁺ ions. CH₃COOH (acetic/ethanoic acid) is a weak acid — it ionises only partially, producing fewer H⁺ ions.
Since pH depends on H⁺ ion concentration (higher H⁺ → lower pH), equal moles of HCl give a lower pH than equal moles of CH₃COOH at the same concentration.
The concept is degree of ionisation (strong vs. weak acids). Equal moles do not mean equal H⁺ ions — it depends on how completely the acid ionises.
Source: Chapter 2, Section 2.3; Chapter 4, Section 4.4.2
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Explanation
- Examiners want three things: (1) identify the correct student, (2) define strong vs. weak acid using ionisation, (3) link it to pH/H⁺ concentration. Each point is roughly 1 mark.
- Key phrase to use: "completely ionised" for HCl vs. "partially ionised / weak acid" for CH₃COOH — the textbook uses exactly this language in Section 4.4.2.
- Do not say Student B is "completely wrong" without explaining why equal moles ≠ equal H⁺; that explanation is the core of the answer.