Why is a solution of ammonium acetate approximately neutral?

• A. Because both ions are strong acids.
• B. Because Ka(NH4+) ≈ Kb(AcO-), hydrolysis cancels.
• C. Because both ions are strong bases.
• D. Because the salt does not hydrolyze.

Answer: (B)

This question centers on how salts from a weak acid and a weak base affect pH. In water, the ammonium ion acts as a weak acid, donating H+ through NH4+ + H2O ⇌ NH3 + H3O+, with Ka about 5.6 × 10^-10. The acetate ion acts as a weak base, accepting a proton through AcO- + H2O ⇌ HOAc + OH-, with Kb around Kw/Ka(HOAc) ≈ 1.0 × 10^-14 / 1.8 × 10^-5 ≈ 5.6 × 10^-10. Because these two hydrolysis processes produce hydronium and hydroxide ions at nearly the same rate, their effects cancel each other out, leaving the solution essentially neutral. If the cation’s acidity were stronger or the anion’s basicity stronger, the pH would skew acidic or basic accordingly. Here, the close equality of Ka for NH4+ and Kb for AcO- explains why ammonium acetate is approximately neutral.