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Difficulty: MediumSalt Hydrolysis and Solution Acidity/Alkalinity

When potassium ethanoate, CH3COOKCH_3COOK, is dissolved in distilled water, the resulting solution has a pH greater than 7. Which ion is responsible for this alkalinity by undergoing hydrolysis?

  1. The ethanoate ion, CH3COO−CH_3COO^-Answer
  2. B
    The potassium ion, K+K^+
  3. C
    The hydrogen ion, H+H^+
  4. D
    The hydroxide ion, OH−OH^-

Answer

The ethanoate ion, CH3COO−CH_3COO^-
The ethanoate ion (CH3COO−CH_3COO^-) is the conjugate base of the weak acid ethanoic acid (CH3COOHCH_3COOH). When potassium ethanoate dissolves in water, CH3COO−CH_3COO^- hydrolyzes by accepting a proton from water to form CH3COOHCH_3COOH and releasing OH−OH^- ions into the solution: CH3COO−(aq)+H2O(l)⇌CH3COOH(aq)+OH−(aq)CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq). The accumulation of free hydroxide ions makes the solution alkaline (pH>7pH > 7).

Step-by-Step Solution

1
Identify the ions formed when potassium ethanoate dissolves in water.
Potassium ethanoate dissociates completely into potassium ions (K+K^+) and ethanoate ions (CH3COO−CH_3COO^-).
Soluble ionic salts completely dissociate into their constituent cations and anions in aqueous solution.
2
Determine which ion reacts with water (hydrolyzes).
Potassium (K+K^+) is derived from a strong base (KOHKOH) and does not hydrolyze. Ethanoate (CH3COO−CH_3COO^-) is derived from a weak acid (CH3COOHCH_3COOH) and reacts with water.
Anions derived from weak acids act as weak conjugate bases and undergo anion hydrolysis.
3
Write the hydrolysis equation to identify the species giving rise to alkalinity.
CH3COO−(aq)+H2O(l)⇌CH3COOH(aq)+OH−(aq)CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)
Hydrolysis of ethanoate generates free OH−OH^- ions, making the solution basic (pH>7pH > 7).

Key Concept

Anion hydrolysis of salts formed from a weak acid and a strong base
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Topics

Salt Hydrolysis and Solution Acidity/Alkalinity

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