An aqueous solution at 25 degree C is 0.1 M in both Mg^2+ and Pb^2+ ions (for ex

Question

An aqueous solution at 25 degree C is 0.1 M in both Mg^2+ and Pb^2+ ions (for example as the soluble nitrates). We wish to separate the two kinds of metal ions by taking advantage of the different solubilities of their oxalates, MgC_2O_4 and PbC_2O_4. Oxalate can be thought of as two carboxylic acid groups joined back to back: The respective K_sp values are: Which ion will precipitate first as the oxalate ion concentration is raised: Mg^2+ or Pb^2+? What is the highest possible oxalate ion concentration that allows only one solid oxalate salt to be present at equilibrium? What is the concentration of the less soluble ion remaining in solution under the conditions of part (b)?

Explanation / Answer

a) Ksp of PbC2O4 is lower than MgC2O4, hence PbC2O4 will precipitate out first from the solution.

b) Ksp for MgC2O4 = [Mg2+][C2O4^2-]

8.6 x 10^-5 = [0.1][C2O4^2-]

[C2O4^2-] = 8.6 x 10^-4 M

So at oxalate concentration of 8.6 x 10^-4 M only MgC2O4 would be present at equilibrium

c) Ksp for PbC2O4 = [Pb2+][C2O4^2-]

2.7 x 10^-11 = [Pb2+][8.6 x 10^-4]

[Pb2+] = 3.14 x 10^-8 M remains in solution

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