Question

CH 17 #20

A solution of Na2CO3 is added dropwise to a solution that contains 1.10×10^?2M Fe2+ and 1.54×10^?2M Cd2+. What concentration of CO32? is need to initiate precipitation? Neglect any volume changes during the addition.

Express your answer with the appropriate units.

The solubility-product constants, Ksp, at 25 ?C for two compounds [iron(II) carbonate, FeCO3, and cadmium(II) carbonate, CdCO3] are given by the table

Substance	Ksp
FeCO3	2.10×10?11
CdCO3	1.80×10?14

The solubility-product constants, K sp , at 25 ? C for two compounds [iron(II) carbonate, FeC O 3 , and cadmium(II) carbonate, CdC O 3 ] are given by the table Substance K sp FeC O 3 2.10× 10 ?11 CdC O 3 1.80× 10 ?14

Answer 1

Ksp of FeCO3 = 2.10 x 10^-11

Ksp of CdCO3 = 1.80 x 10^-14

here Ksp of CdCO3 is less than FeCO3. so

CdCO3 will precipitate first.

CdCO3   ---------------> Cd+2   +   CO32-

Ksp = [Cd+2] [CO32-]

1.80 x 10^-14 = (1.54 x 10^-2) [CO32-]

[CO32-] = 1.17 x 10^-12 M

concentration of [CO32-] = 1.17 x 10^-12 M