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Difficulty: Very hardSalt Hydrolysis and Solution Acidity/Alkalinity

An aqueous solution of ammonium ethanoate (CH3COONH4CH_3COONH_4) is approximately neutral with a pH close to 7 because neither the ammonium cation (NH4+NH_4^+) nor the ethanoate anion (CH3COO−CH_3COO^-) undergoes hydrolysis in water.

Answer: Answer

Answer

The statement is False. Both the cation (NH4+NH_4^+) and anion (CH3COO−CH_3COO^-) undergo hydrolysis in water, but because the acid dissociation constant of the parent weak acid (KaK_a) equals the base dissociation constant of the parent weak base (KbK_b), the concentrations of generated hydronium and hydroxide ions are equal, producing a neutral solution.
The statement incorrectly attributes the neutral pH of aqueous ammonium ethanoate to an absence of hydrolysis. In reality, both the conjugate acid (NH4+NH_4^+) and conjugate base (CH3COO−CH_3COO^-) hydrolyze simultaneously; neutrality arises solely because KaK_a of ethanoic acid equals KbK_b of ammonia.

Step-by-Step Solution

1
Identify the parent acid and parent base of ammonium ethanoate (CH3COONH4CH_3COONH_4).
Ammonium ethanoate is derived from ethanoic acid (CH3COOHCH_3COOH), a weak acid, and aqueous ammonia (NH3NH_3), a weak base.
Salts formed from weak acids and weak bases yield conjugate species that are strong enough to react with water.
2
Analyze the hydrolysis behavior of both ions in aqueous solution.
Cation hydrolysis: NH4++H2O⇌NH3+H3O+NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+. Anion hydrolysis: CH3COO−+H2O⇌CH3COOH+OH−CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-.
Both NH4+NH_4^+ (weak acid) and CH3COO−CH_3COO^- (weak base) hydrolyze simultaneously in water.
3
Determine the net effect of mutual hydrolysis on solution pH.
Since Ka(CH3COOH)=1.8×10−5K_a(CH_3COOH) = 1.8 \times 10^{-5} and Kb(NH3)=1.8×10−5K_b(NH_3) = 1.8 \times 10^{-5}, [H3O+]=[OH−][H_3O^+] = [OH^-], leading to pH≈7.0\text{pH} \approx 7.0.
Equal ionization constants produce balanced amounts of hydronium and hydroxide ions, resulting in a neutral solution despite significant hydrolysis of both ions.

Key Concept

Hydrolysis of salts derived from a weak acid and a weak base
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Topics

Salt Hydrolysis and Solution Acidity/Alkalinity

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