Question

A solution contains 0.002 M Ag+ and 1.6 x 10-5 M Zn2+, and the ions need to be separated. For this purpose begin mathsize 11px style CO subscript 3 superscript 2 minus end superscript end style is added to the solution so that both Ag2CO3 (Ksp = 8.1 x 10-12) and ZnCO3 (Ksp = 1.0 x 10-10) can precipitate from the solution. What will the concentration of begin mathsize 11px style CO subscript 3 superscript 2 minus end superscript end stylebe just before the first salt starts to precipitate.
Please explain

Answer 1

Your post is hard to read because of the "mathsize" and all that follows. Apparently you are trying to communicate carbonate ion which is written as CO3^2-. Technically that isn't right, of course, but everybody understands that the 3 is a subscript and the 2- is a superscript. Anyway, what you have here is a solution of two salts. You want to know what the CO3^2- must be to ppt the first salt. The plan is to calculate (CO3^2-) required to ppt each salt. The one requiring the smallest amount of CO3^2- will be the one that ppts first.
.......................ZnCO3 ==> Zn^2+ + CO3^2-
I......................solid...............0..............0
C.....................solid...............x..............x
E......................solid...............x..............x
Ksp = (Zn^2+)(CO3^2-)
You know Ksp and Zn^2+ from the problem. Calculate (CO3^-); i.e., the minimum amount necessary to ppt the first molecule of ZnCO3.
Now you do the same thing for Ag2CO3. You know (Ag^2+) from the problem. You calculate the minimum amount of CO3^2- needed to start precipitating Ag2CO3. The salt that requires the smallest amount of CO3^2- will be the one that is ready to ppt when the next CO3^2- ion is added.
Post your work if you get stuck.