chemistry-pH of solution (9.20)

Question

Indicate what happens to the pH of the solution when a solution of K2S in water with a concentration greater than 10^-3 M is added to pure water? Ka(H2S)=8.9 ・10^-18 and Ka(HS^-)=1.2 ・10^-13

A. The substance does not affect the pH. The pH remains equal to 7.0.

B. The substance behaves like a weak acid. The pH is less than 7.0.

C. The substance behaves like a weak base. The pH is greater than 7.0. (the answer)

D. The substance behaves amphiprotic. The pH can be greater or less than 7.0.

Answer 1

K2S will behave as a weak base. No calculation is required. K⁺ comes from a strong base (KOH). It is the conjugate acid of a strong base, so it is extremely weak. Thus, K⁺ will not effect the pH. S^2- comes from an extremely weak acid (HS^-). It is the conjugate base of a weak acid, so it is a base that is strong enough to effect the pH. Therefore, K2S will act as a weak base in solution and have a pH greater than 7.