An aqueous solution contains [Ba²⁺] = 5.0 × 10^-5 M,[Ag⁺] = 3.0 × 10^-5 M,and [Zn²⁺] = 2.0 × 10^-7 M.Sodium oxalate is slowly added so that [C₂O₄^2-] increases.
Salt BaC₂O₄ ZnC₂O₄ Ag₂C₂O₄
K_sp 1.5 × 10^-8 1.35 × 10^-9 1.1 × 10^-11
What is the concentration of the first cation to precipitate when the second cation just begins to precipitate?

Question

Quizplus · Accepted Answer

The first cation to precipitate is the one with the lowest Ksp value when compared to the ion product (Q) of the solution for that cation. In this case, Ag2C2O4 (Ksp = 1.1 × 10^-11) will precipitate first because it has the lowest Ksp, indicating it is the least soluble. When Ag+ begins to precipitate, the concentration of Ag+ is determined by the Ksp of Ag2C2O4 and the concentration of oxalate ions. Using the formula Ksp = [Ag+]^2[Oxalate^2-], and solving for [Ag+] when oxalate concentration allows the second cation (BaC2O4, with the next lowest Ksp of 1.5 × 10^-8) to begin precipitating, gives the concentration of Ag+ at the point just before BaC2O4 starts to precipitate. This calculation involves setting the ion product equal to the Ksp and solving for the concentration of the cation, which in this scenario leads to the answer provided.

An Aqueous Solution Contains [Ba2+] = 5

An Aqueous Solution Contains [Ba²⁺] = 5