Question

It is of interest to decide if an analytical separation of the metal ions can be effected by selective precipitation of carbonates from a solution that is 9.88×10^-2 M in Fe²⁺ and 0.106 M in Ni²⁺.

NiCO3	Ksp = 6.60×10-9
FeCO3	Ksp = 3.50×10-11

To analyze this problem, answer the following questions.

(1) What carbonate concentration is needed to precipitate 99.9% of the metal that forms the least soluble carbonate?

M

(2) When 99.9% of the least soluble carbonate has precipitated, will all of the metal that forms the more soluble carbonate still remain in solution?

(3) What is the upper limit on the carbonate ion concentration if the more soluble compound is not to precipitate? M

(4) If the [CO₃^2-] is at this upper limit, what percentage of the metal that forms the least soluble carbonate remains in solution? %

Answer 1

Leas tsoluble = FeCO3, sicne it has lower Ksp

Q1

find CO3-2 required to precipitate hte least soluble

Ksp = [Fe+2][CO3-2]

99.99% of Fe+2 = (100-99.99) = 0.01 % left

0.01/100*(9.88*10^-2) = 0.00000988 M of Fe+2 left in solution

3.5*10^-11 = (0.00000988)([CO3-2]

[CO3-2] = (3.5*10^-11)/(0.00000988) = 0.000003542 M = 3.542*10^-6 M

Q2

find the other metal at this point

Ksp = [Ni+2][CO3-2]

(6.6*10^-9) = [Ni2+] ( 3.542*10^-6)

[Ni2+] = (6.6*10^-9) / (3.542*10^-6) = 0.0018633

therefore, not all the Nickel has precipitated

Q3

upper limit so the soluble compound will not precipitate

Ksp =[Ni2+][CO3-2]

(6.6*10^-9) / (0.106) = [CO3-2]

[CO3-] = 6.2264*10^-8 M then Ni2+ wont form preciptiate

Q4

if CO3-2 is at this value, find metal of leaast soluble

Ksp = [Fe+2][CO3-2]

3.5*10^-11 = [Fe+2] (  6.2264*10^-8)

[Fe+2] = (3.5*10^-11) / (  6.2264*10^-8) = 0.0005621 M

% remains = (0.0005621)/(9.88*10^-2)*100 =0.568 %