Question

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

Part A

A solution of Na2CO3 is added dropwise to a solution that contains 1.04×10⁻²M Fe2+ and 1.50×10⁻²M Cd2+. What concentration of CO32− is need to initiate precipitation? Neglect any volume changes during the addition.

Express the molar concentration numerically.

Part B

In the solution from Part A, what will the concentration of CO32− be when Fe2+ begins to precipitate?

Express the molar concentration numerically.

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

Answer 1

Part A

Dissolution equillibrium of FeCO3 is

FeCO3(s) <------> Fe²⁺(aq) + CO3^2-(aq)

Ksp = [Fe²⁺][CO3^2-]= 2.10*10^-11

given concentration of Fe²⁺ = 0.0104M

So,

0.0104M * [CO3^2-] = 2.10*10^-11M²

[CO3^2-]=2.10*10^-11M²/0.0104M

= 2.02*10^-9

So, when [CO3^2-] reaches 2.02*10^-9M , FeCO3 start to precipitate

dissolution equillibrium of CdCO3 is

CdCO3(s) <-----> Cd²⁺(aq) + CO3^2-(aq)

Ksp = [ Cd²⁺][CO3^2-]=1.80*10^-14

given concentration of Cd²⁺ = 0.015M

So,

0.015*[CO3^2-] = 1.80*10^-14M²

[CO3^2-] = 1.20*10^-12M

So, when [CO3^2-] reaches 1.20*10^-12M , CdCO3 start to precipitate

therfore,

CdCO3 will precipitate first

Part B

When [ CO3^2-] reaches 2.02*10^-9M , FeCO3 start to precipitate

[Cd²⁺][CO3^2-] = 1.80*10^-14M²

When , [CO3^2-] = 2.02*10^-9M

[Cd²⁺]*2.02*10^-9M = 1.80*10^-14M²

[Cd²⁺] = 1.22*10^-5M

So, when FeCO3 start to precipitate

[Cd²⁺] = 1.22*10^-5M