Equal concentrations of hydrochloric acid (HCl) and acetic acid (CH₃COOH) are taken in separate beakers. How would their pH values compare, and what does this comparison reveal about the strength of the two acids?
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Model Answer
At equal concentrations, HCl has a lower pH than CH₃COOH (acetic acid).
This is because HCl is a strong acid — it ionises completely in water, producing more H⁺ ions. Acetic acid is a weak acid — it ionises only partially, producing fewer H⁺ ions. Since higher H⁺ concentration means lower pH, HCl has a lower pH than acetic acid at the same concentration.
Explanation
- Examiners look for two things: (1) stating which pH is lower, and (2) linking it to complete vs. partial ionisation (strong vs. weak acid).
- Key phrase from the textbook: "Acids that give rise to more H⁺ ions are said to be strong acids, and acids that give less H⁺ ions are said to be weak acids."
- Don't just say "HCl is stronger" — explain why in terms of H⁺ ions produced.
Source: Chapter 2, Section 2.3; Chapter 4, Section 4.4.2