Salt Solutions and Hydrolysis

Dissolve sodium bicarbonate in water and the pH climbs above 7, even though nothing basic was poured in. The trick is that bicarbonate, HCO3-, is the conjugate base of a weak acid. Conjugate bases of weak acids are decent proton grabbers: they pull H+ off water, HCO3- + H2O → H2CO3 + OH-, leaving extra OH- behind. That is hydrolysis: an ion reacting with water to generate H3O+ or OH-. The strength of this pull is measured by Kb = Kw/Ka, where Ka belongs to the parent weak acid. A smaller Ka means a stronger conjugate base and a more basic solution.

The same logic runs in reverse for cations. NH4+, the conjugate acid of the weak base NH3, donates a proton to water, NH4+ + H2O → NH3 + H3O+, making the solution acidic. So to predict a salt's solution, look at both pieces. NaCl splits into Na+ and Cl-, spectators from a strong base and a strong acid, so the solution stays neutral. NaCH3COO gives acetate, the conjugate base of a weak acid, so it turns basic. NH4Cl gives NH4+, conjugate acid of a weak base, paired with Cl-, a spectator, so the solution turns acidic. The acid-base origin of the salt is irrelevant; what matters is whether an ion is conjugate to something weak.

When both ions can hydrolyze, like in NH4CH3COO, you compare Ka of the cation to Kb of the anion: whichever reaction goes further sets the direction of the pH shift. This sets up buffers next, where a weak acid and its conjugate base coexist deliberately, using the same hydrolysis equilibria to resist pH change.