Question

what is the solubility of ZnCO3 at 25C in a buffer solution with pH of 9.70?

Answer 1

Let me guide you in how you'll do it:

......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.
alpha2 = (k1k2) / [H+]² + k1(H⁺) + k1k2
You have k1 and k2 (The ionization constant for H2CO3 and HCO3, look for them) as well [H+] [from the pH]. Solve for alpha 2.
Ksp = [Zn^2+][CO3^2-]
(Zn^2+) = x
(CO3^2-) = alpha2 * x
Solve for x - solubility, the Ksp for ZnCO3 you may find it online. so, solve now for X.

Post your work if you get stuck.

Hope this helps