Question

The solubility of PbSO₄ (Ksp = 6.3 x 10⁻⁷) decreases with adding which of the following?

a. NaCro₄ (Ksp of PbCrO₄ = 2.8 x 10⁻¹¹)
b. PbF₂ (Ksp of PbF₂ = 3.6 x 10⁻⁸)
c. NO₃⁻
d. Na₂S (Ksp of PbS = 3 x 10⁻²⁸)

Answer 1

The solubility of PbSO4 decreases after adding Na2S due to the common ion effect, which leads to the formation of insoluble PbS precipitate, thereby decreasing Pb2+ ions in solution and the solubility of PbSO4.

Step-by-step explanation:

The solubility of PbSO4 decreases with the addition of substances that can provide either the Pb2+ ion or the SO42- ion, due to the common ion effect. This effect is a shift in equilibrium that suppresses the solubility of a sparingly soluble salt when a common ion is added to the solution.

Adding Na2S (which provides the S2- ion) would decrease the solubility of PbSO4 significantly since PbS has a very low Ksp (3 x 10-28). The S2- ion from Na2S would combine with Pb2+ ions to form PbS, thereby removing Pb2+ ions from the solution and shifting the equilibrium to further decrease the solubility of PbSO4.

Therefore, the solubility of PbSO4 decreases with the addition of Na2S due to the common ion effect.