[Sarex]

Hy guy, I'd like to ask you some help in an exercise please... it's about solubility linked to pH.
Here it is: calculate the solubility (mg/L) of CaCO₃  in a tampon solution pH=8,5,
knowing that : Ksp CaCO3 = 1,6*10^-8; Ka1 of H2CO3 = 3,3*10^-7; Ka2= 5,0*10^-11.
I know that the solubility of a substance increases in an acid solution and I know how to solve exercises in the presence of an acid; but in this case, the environment is basic (pH 8,5) and a really don't know how to move...
Thank you in advance to anyone who will stop and help 

Mod Edit: fixed capitals in Subject Title to be more in line with forum rules

[AWK]

Forget Kas. Use simply Ksp.

[Borek]

Forget Kas. Use simply Ksp.

At pH 8.5 most CO₃^2- is protonated to HCO₃^-. Finding what fraction of the carbonate is still in the form of CO₃^2- is a key to the success here.

[AWK]

Assuming complete dissociation of carbonic acid I did very large error (underestimation more than 800 %). Fortunately, Borek watched.