794) Substance X is amphiprotic, which means it can either accept or donate a proton (acting like an acid or a base). An example is HCO3-. It can accept a proton (as a base) to become H2CO3 or lose a proton (as an acid) to become CO3(2-). Notice this question only gave you the pKa - the acid condition. There's no way of predicting the pKb for the same molecule (remember the pKa + pKb = 14 rule ONLY applies to conjugate PAIRS). Without knowing the pKb, it impossible to predict the pH

811) HCO3- is a better base than acid, as it has a pKa of 10.25, and a pKb of 7.63, which makes the solution behaves more like a base than an acid.

838) picture adding 0.1mol of HCl to 1 litre of both buffers. In buffer B, all of the acetate ions will be protonated (0.08moles), which used up all of its buffering capacity. However, in buffer A, you'd still have 0.7moles of acetate ion left. Thus solution A has a greater buffering capacity