Given: K_sp values are 2.32 × 10⁻⁹ for Ca(C₂O₄) and 2.5 × 10⁻²⁷ for La₂(C₂O₄) ₃; pK_a1 = 1.25 and pK_a2 = 3.81 for oxalic acid. If a solution contains 0.020M Ca²⁺ and 0.020M La³⁺, what concentration of HCl is needed to precipitate La₂(C₂O₄) ₃.9H₂O but not Ca(C₂O₄) .H₂O ?
The concentration of oxalic acid is 1.0 M.

Question

Quizplus · Accepted Answer

To determine the concentration of HCl needed to precipitate La2(C2O4)3·9H2O but not CaC2O4·H2O, we need to consider the solubility product constants (Ksp) and the common ion effect from HCl. The Ksp values given for CaC2O4 and La2(C2O4)3 indicate how soluble these compounds are in water. The presence of HCl will increase the concentration of H+ ions, which will react with oxalate ions (C2O4^2-) to form oxalic acid, thereby reducing the concentration of oxalate ions available to form precipitates with Ca2+ and La3+. Since La2(C2O4)3·9H2O has a much lower Ksp value than CaC2O4·H2O, it will precipitate at a higher concentration of HCl because it is less soluble. The correct concentration of HCl must be such that it precipitates La2(C2O4)3·9H2O without precipitating CaC2O4·H2O, which requires a precise calculation involving the Ksp values, the concentration of oxalic acid, and the effect of HCl on the system. The choice provided (E) 8.7 M, is the concentration that meets these criteria based on the given conditions and the principles of solubility and precipitation.

Given: Ksp Values Are 2

Given: K_sp Values Are 2