posted by Jim on .
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.
What are the Ksp values for NiCO3 and CuCO3?
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.