Question

a solution contains .25M Ni(NO3)2 and .25 M Cu(NO3)2. Can the metal ions be separated by slowly adding Na2CO3?Assume that for successful separation 99% of the metal ion must be precipitated before the other metal ion begins to precipitate, and assume no vlume change on addition of Na2CO3.

Answer 1

What are the Ksp values for NiCO3 and CuCO3?

Answer 2

I looked up some Ksp values for CuCO3 and NiCO3 and found the following:
Ksp NiCO3 = 1.4 x 10^-7
Ksp CuCO3 = 2.5 x 10^-10
You should confirm those numbers.

(Cu^+2)(CO3^-2)...(2.5 x 10^-10
--------------- = ------------- =0.00179
(Ni^+)(CO3^-2)....(1.4 x 10^-7)

Therefore,(Cu^+2)/(Ni^+2) = 0.00179 and
(Cu^+2) = 0.00179*(Ni^+2).
If all of the Cu^+ is pptd, then before the next molecule of Na2CO3 is added, the (Ni^+2) = 0.25 and (Cu^+2) = 0.00179 x (0.25) = 0.000448 in solution. Thus that is the unpptd Cu^+2. [Note: It is better to be using moles at this point; however, since Na2CO3 is a solid we are not diluting the solution, and molarity works ok.]
Thus percent Cu^+2 pptd before the first molecule NiCO3 ppts is
0.000448/0.25)*100 = 0.18% which means that 99.8% pptd. Thus, the two can be separated. Check my work. Check my thinking.