A solution of Na2CO3 is added dropwise to a solution that is 0.010 M in Fe^2+ an

Question

A solution of Na2CO3 is added dropwise to a solution that is 0.010 M in Fe^2+ and 0.010 M in Cd^2+. The Ksp for FeCO3 is 2.10*10^-11. The Ksp for CdCO3 is 1.8*10^-14

A) Which Ion precipitates first?

B) What concentration of CO3^2- is necessary for precipitation to begin? (Neglect volume changes)

C) What is the concentration of the cation that precipitated first when the second cation begins to precipitate? (Neglect volume changes)

D) The criterion for effective seperation of the less soluable ion is if only 0.001% (or less) of it remains in solution just before the second ion beins to precipitate. Determine if Na2CO3 can effectively seperate Fe^2+ and Cd^2+

Explanation / Answer

a) Precipitate first the one with lower Ksp value.
CdCO3

b) Ksp = [Cd2+][CO32-]
[CO32-] = Ksp/[Cd2+]
[CO32-] = 1.8x10-14/0.010 M
[CO32-] = 1.8x10-12

c) Ksp = [Fe2+][CO32-]
[CO32-] = Ksp/[Fe2+]
[CO32-] = 2.1x10-11/0.010 M
[CO32-] = 2.1x10-9 M

[Cd2+] = Ksp/[CO32-]
[Cd2+] = 1.8x10-14/2.1x10-9 M
[Cd2+] = 1.8x10-14/2.1x10-9 M
[Cd2+] = 8.6x10-6 M

d) Calculate 0.001% of original concentration:
[Cd2+] = 0.010 M . (0.001/100)
[Cd2+] = 1x10-7 M

Since:
8.6x10-6 M > 1x10-7 M.
Fe2+ and Cd2+ can NOT be effectively separated with Na2CO3.

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