Why is the aqueous solution of ammonium acetate (CH3COONH4), a salt of a weak acid and a weak base, nearly neutral (pH ≈ 7.0) at 298 K?

Answer and explanation

Correct answer: B. Both ions hydrolyze equally since Ka of acetic acid matches Kb of ammonium hydroxide (~1.8 * 10^-5)

  1. Step 1: Ammonium acetate is formed from a weak acid () and a weak base ().
  2. Step 2: In water, both the cation (, cationic hydrolysis producing ) and the anion (, anionic hydrolysis producing ) undergo hydrolysis.
  3. Step 3: The pH of a salt of weak acid and weak base is given by: .
  4. Step 4: For acetic acid and ammonium hydroxide, , so .
  5. Step 5: Substituting gives: , making the solution neutral.
Answer
Because both ions undergo hydrolysis, and the dissociation constant of acetic acid (Ka = 1.8 10^-5) is almost identical to the dissociation constant of ammonium hydroxide (Kb = 1.8 10^-5)
Why not the others: 'Because neither ion undergoes hydrolysis...' is false; salts of strong acids and strong bases (e.g., ) show no hydrolysis; 'Because it precipitates as an insoluble crystal...' is false as it is freely soluble; 'Because atmospheric oxygen neutralizes...' is fabricated.

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