Which is the best acid-base pair to use in the preparation of a buffer with pH = 10.5? (a) HOI and OI- (Ka = 2.0 x 10^-11) (b) HNO2 and NO2- (Ka = 4.5 x 10^-4) (c) HIO3 and IO3- (Ka = 1.7 x 10^-1) (d) H2PO4^- and HPO4^2- (Ka = 6.2 x 10^-8)

A buffer solution is a system that resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. The effectiveness of a buffer is determined by the pKa of the acid and the desired pH, which should be close to the pKa for optimal buffering capacity.

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the acid and the ratio of the concentrations of the conjugate base to the weak acid. It is expressed as pH = pKa + log([A-]/[HA]). This equation is crucial for determining the appropriate acid-base pair to achieve a specific pH, such as 10.5 in this case.

The acid dissociation constant (Ka) quantifies the strength of an acid in solution, indicating how well it donates protons (H+) to the solution. A higher Ka value corresponds to a stronger acid, which is more effective at maintaining the desired pH in a buffer. When selecting an acid-base pair for a buffer, the pKa (the negative logarithm of Ka) should be close to the target pH for optimal performance.