When comparing the acidity of ammonium ion (NH4+) and ammonia (NH3), we need to understand their properties in terms of acids and bases.
NH4+ is indeed the stronger acid when we compare these two species. This is because NH4+ can donate a proton (H+) to water, forming ammonia and hydronium ion (H3O+):
NH4+ + H2O ⇌ NH3 + H3O+
On the other hand, ammonia (NH3) acts as a weak base. It can accept a proton from water, but it does not readily donate a proton. Thus, under normal conditions, ammonia is not considered an acid:
NH3 + H2O ⇌ NH4+ + OH–
The tendency of NH4+ to donate a proton and increase the concentration of hydronium ions in solution makes it a stronger acid compared to ammonia, which does not donate protons readily.
In summary, NH4+ is a stronger acid than NH3 because it has a greater ability to donate protons, thereby increasing the acidity of the solution.