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) Ka(NH4+) = 5.6 x 10-10F–(aq) + H2O(l) ↔ HF(aq) + OH–(aq) Kb(F–) = 1.4 x 10-11
Since Ka(NH4+) > Kb (F–), the reaction of ammonia with water is more favorable. Therefore, the resulting solution is slightly acidic.