Question

A solution of Na2CO3 is added dropwise to a solution that contains 1.15×10−2 M Fe2+ and 1.58×10−2 M Cd2+. What concentration of CO32− is need to initiate precipitation? Neglect any volume changes during the addition.

Answer 1

As we know that the precipitation will initiate when the solubility product (Ksp) is less than the ionic product (Q) of the sparingly soluble salt. More specifically,

Ksp > Q => Solution is supersaturated. Hence no precipitation occur.

Ksp = Q => Solution is saturated. Hence no precipation occur.

Ksp < Q => Solution is formed precipate. This is the condition for precipitation.

Here we know the Ksp of FeCO₃ and CdCO₃ which is

Ksp for FeCO₃ = 2.1x10^-11= [Fe²⁺][CO₃^2-] .......................(1)

Ksp for CdCO₃ = 1.8x10^-14 = [Cd²⁺][CO₃^2-] ......................(2)

Given,

[Fe²⁺] = 1.15x10⁻² M

[Cd²⁺] = 1.58x10⁻² M

Putting these values in (1) and (2), we will get

(1) CO₃^2- concentration required to initiate precipitation of FeCO₃

[CO₃^2-] > Ksp for FeCO₃ / [Fe²⁺] > 2.1x10^-11 / 1.15x10⁻² > 1.83x10⁻⁹ M

(2) CO₃^2- concentration required to initiate precipitation of CdCO₃

[CO₃^2-] > Ksp for CdCO₃ / [Cd²⁺] > 1.8x10^-14 / 1.58x10⁻² > 1.14x10⁻¹² M

As the solution contains both the ions, So, to initiate the precipitation we required the less concentration of CO₃^2-.

From calculations (1) and (2), the CO₃^2- concentration is less in case of (2) that is [CO₃^2-] >1.14x10⁻¹² M. Hence, we require the slighly greater concentration of CO₃^2- ion than 1.14x10⁻¹² M.