Why is hydronium ion a stronger acid than water and hydroxide ion a stronger base than water?
Compare the acid strength of H3O+ to H2O. These are binary acids whose charge differs by +1. Charge always dominates acid strength for otherwise similar species. The cation is the stronger acid because of the charge repulsion between the cation and the H+ it is donating. Compare the acid strength of OH and H2O. The reasoning is similar to the above: since OH is more negatively charged, it wants to hold on to its H+ much more strongly than the neutral H2O. Thus, H2O is a better H+ donor and a better acid. Base strength is inversely related to acid strength: stronger acids are weaker bases. Thus, since H2O is the stronger acid, it must be the weaker base.
