Q1. [3] deep exam-ready
Consider the following salts: sodium chloride, sodium carbonate, ammonium chloride, and sodium acetate.
(a) Which of these will have a pH equal to 7, less than 7, and greater than 7? Justify based on the acid and base used to form each salt.
(b) State the general rule for predicting the pH nature of a salt.
Generated by claude-sonnet-4-6 · 2026-06-17 12:39 · grounding rag
Model Answer
(a) pH classification of the four salts:
- Sodium chloride (NaCl): pH = 7 (neutral). Formed from strong acid HCl and strong base NaOH.
- Ammonium chloride (NH₄Cl): pH < 7 (acidic). Formed from strong acid HCl and weak base NH₄OH.
- Sodium carbonate (Na₂CO₃): pH > 7 (basic). Formed from weak acid H₂CO₃ and strong base NaOH.
- Sodium acetate (CH₃COONa): pH > 7 (basic). Formed from weak acid CH₃COOH (acetic acid) and strong base NaOH.
(b) General rule:
- Salt of strong acid + strong base → pH = 7 (neutral)
- Salt of strong acid + weak base → pH < 7 (acidic)
- Salt of strong base + weak acid → pH > 7 (basic)
Source: Chapter 2, Section 2.4.2
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Explanation
- The examiner wants you to name the acid and base forming each salt and link it directly to the pH — this earns the justification marks.
- For part (b), state the rule as three clean bullet points; this is the textbook statement and should be memorised exactly.
- Acetic acid (ethanoic acid) is a weak acid — confirmed in Chapter 4; carbonic acid (H₂CO₃) is also weak. NH₄OH is a weak base. NaOH and HCl are strong — this reasoning is what examiners check.