Question

Determine the minimum concentration of the precipitating agent on the right to cause precipitation of the cation from the solution on the left.

4.0×10⁻² M Ba(NO3)2;NaF

8.5×10⁻² M CaI2;K2SO4

Answer 1

1)

Solubility equillibrium of BaF2 is as follows

BaF2(s) <-------> Ba²⁺(aq) + 2F^-(aq)

Ksp = [Ba²⁺] [F^-]² = 1.8×10^-7

substituting given concentration of Ba²⁺ in Ksp expression

4.0×10^-2M × [F^-]² = 1.8×10^-7M³

[F^-]² = 4.5 × 10^-8M²

[F^-] = 2.12 × 10^-4M

Therefore,

Minimum concentration of NaF required to start precipitation of Ba²⁺ = 2.12×10^-4M

2)

Solubility equillibrium of CaSO4 is as follows

CaSO4(s) <-------> Ca²⁺(aq) + SO4^2-(aq)

Ksp = [Ca²⁺][SO4^2-] = 4.93×10^-5

substituting given concentration of Ca²⁺

8.5×10^-2M × [SO4^2-] = 4.93 ×10^-5M²

[SO4^2-] = 5.8 ×10^-4M

Therefore,

minimum concentration of K2SO4 required to srart precipitation of Ca²⁺ = 5.8×10^-4M