The pH of a 0.50 M solution of the weak monoprotic acid HA is 4.76. The value of K_a is

Question

Quizplus · Accepted Answer

The pH of the solution is 4.76, which means the concentration of H+ ions is 10^-4.76. For a weak acid HA dissociating into H+ and A-, the Ka (acid dissociation constant) can be calculated using the formula Ka = [H+][A-]/[HA], where [HA] is the initial concentration of the acid. Given that the solution is 0.50 M and assuming complete dissociation (which is not true but simplifies the calculation), the concentration of H+ (and thus A-) would be approximately 10^-4.76. However, since the acid is weak and does not dissociate completely, the actual concentration of A- will be less, and [HA] will be slightly less than 0.50 M. The pH given allows us to directly calculate the Ka without needing the exact concentrations of A- and HA at equilibrium because the pH implies the concentration of H+ ions in the solution. Using the pH to find the H+ concentration, Ka = [H+]^2 / [HA] (assuming [H+] ≈ [A-] for weak acids), and since [HA] is approximately the initial concentration (0.50 M), we can simplify this to Ka ≈ [H+]^2 / 0.50. Plugging in the H+ concentration from the pH (10^-4.76), we find that the Ka is closest to the value given in option A, 6.0 × 10^-10, which is a reasonable value for a weak acid's dissociation constant.

The PH of a 0