Question

A solution of Na2CO3 is added dropwise to a solution that contains 1.12×10−2 M Fe2+ and 1.45×10−2 M Cd2+. What concentration of CO32− is need to initiate precipitation? Neglect any volume changes during the addition.

Answer 1

Answer:

The cadmium carbonate (CdCO3) will precipitate first.

We need 6.9 *10^-11 M CO3^2- to initiate precipitation.

Explanation:

Step 1: Given data

The solution contains

 ⇒ 1.12*10^-2 M Fe2+

 ⇒ 1.45*10^−2 M Cd2+

The Ksp of CdCO3 is 10^-12

The Ksp of FeCO3 is 3.13 * 10^-11

Step 2: Calculate concentration of CO3^2- in FeCO3

FeCO3(s) ⇔ Fe2+ + CO3^2-       Ksp = 3.1*10^-11

Ksp = [Fe2+][CO3^2-]

[CO3^2-] = Ksp / [Fe2+]

[CO3^2-] = 3.13*10^-11 / 1.12*10^-2

[CO3^2-] = 2.795 * 10^-9 M

Step 3: Calculate concentration of CO3^2- in CdCO3

CdCO3(s) ⇔Cd^2+ + CO3^2-            Ksp = 10^-12

Ksp = [Cd2][CO3^2-]

[CO3^2-] = Ksp /[Cd^2+]

[CO3^2-] =  10^-12 / 1.45x10^-2

[CO3^2-] =6.9 *10^-11 M

The cadmium carbonate (CdCO3) will precipitate first.

We need 6.9 *10^-11 M CO3^2- to initiate precipitation.