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Difficulty: MediumRelative Strength and Ionization of Acids and Bases

An aqueous solution of 0.01 mol dm−30.01\text{ mol dm}^{-3} ethanoic acid (CH3COOH\text{CH}_3\text{COOH}) exhibits a higher degree of ionization (α\alpha) than a 0.10 mol dm−30.10\text{ mol dm}^{-3} ethanoic acid solution at the same temperature. Which of the following statements correctly explains this observation?

  1. Dilution shifts the ionization equilibrium forward to favor the formation of free ions according to Ostwald's dilution law.Answer
  2. B
    Diluting a weak acid increases its acid dissociation constant (KaK_a), causing it to ionize more completely.
  3. C
    Ethanoic acid converts into a strong acid upon dilution because its concentration decreases significantly.
  4. D
    Concentrated ethanoic acid solutions have a higher percentage ionization because more acid molecules are present per unit volume.

Answer

Dilution shifts the ionization equilibrium forward to favor the formation of free ions according to Ostwald's dilution law.
Dilution increases the volume of the solvent relative to solute molecules, which shifts the position of the dynamic ionization equilibrium toward the side with a greater number of individual particles (ions) to maintain equilibrium, thereby increasing the degree of ionization.

Step-by-Step Solution

1
Write the ionization equilibrium expression for ethanoic acid
CH3COOH(aq)+H2O(l)⇌CH3COO−(aq)+H3O+(aq)\text{CH}_3\text{COOH(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{CH}_3\text{COO}^-(\text{aq}) + \text{H}_3\text{O}^+(\text{aq})
Ethanoic acid is a weak monobasic acid that ionizes partially in aqueous medium.
2
Apply Ostwald's Dilution Law equation for weak electrolytes
α≈KaC\alpha \approx \sqrt{\frac{K_a}{C}}, where α\alpha is degree of ionization, KaK_a is acid dissociation constant, and CC is molar concentration.
As concentration CC decreases (dilution), α\alpha increases, shifting equilibrium toward ion formation.

Key Concept

Ostwald's Dilution Law and Extent of Ionization of Weak Acids
Estimated Time:1m 0s
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Topics

Relative Strength and Ionization of Acids and Bases

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