The chemical reaction for the ammonium ion in water is:
NH4+(aq) + H2O(l) ----> H3O+ (aq) + NH3 (aq) Ka = 5.60x10^-10
The ammonium ion NH4+ in aqueous form reacts with liquid water forming hydronium ion (aqueous) and aqueous ammonia, with a Ka of 5.60x10^-10.