Salts of weak bases and weak acids

Salt solutions containing acidic cations and basic anions such as NH₄F (ammonium fluoride) have two possible reactions:NH₄⁺(aq) + H₂O(l) ↔ H₃O⁺(aq) + NH₃(aq)           K_a(NH₄⁺) = 5.6 x 10-10F^–(aq) + H₂O(l) ↔ HF(aq) + OH^–(aq)                   K_b(F^–) = 1.4 x 10-11

Since K_a(NH₄⁺) > K_b (F^–), the reaction of ammonia with water is more favorable. Therefore, the resulting solution is slightly acidic.