Question

What is the solubility of ZnCO3 at 25 o C in a buffer solution with a pH of 9.74?

Answer 1

......ZnCO3 ==> Zn^2+ + CO3^2-
But CO3^2- + H^+ = HCO3^- and
HCO3^- + H^+ ==> H2CO3
Ksp = (Zn^2+)(CO3^2-)

You can see from the equilibria that the H^+ increases the solubility of ZnCO3 because it adds to the CO3^2- to form H2CO3 (in two steps). You need to find the proportion of CO3^2- since some of it is taken away in the form of HCO3^- and H2CO3.
alpha 2 = (k1k2)/denominator where the denominator is
(H^+)^2 + k1(H^+) + k1k2
You have k1 and k2 as well (H^+) [from the pH]. Solve for alpha 2.
Ksp = (Zn^2+)(CO3^2-)
(Zn^2+) = x
(CO3^2-) = alpha 2*x
Solve for x - solubility
Post your work if you get stuck.

Answer 2

Thank you much