When the compound PbI₂ dissolves, it dissociates as follows;
PbI₂ --> Pb²⁺ + 2I⁻
Molar solubility is the number of moles dissolved in 1 L of solution
A saturated solution is when the maximum amount of solute is dissolved in the solution.
Molar solubility of Iodide when solution is saturated is 2.7 x 10⁻³ mol/L, then solubility of Pb²⁺ is (2.7 x 10⁻³ mol/L) / 2 = 1.35 x 10⁻³ mol/L
ksp is the solubility product constant that can be calculated as follows;
ksp = [Pb²⁺][I⁻]
ksp = (1.35 x 10⁻³ mol/L) x (2.7 x 10⁻³ mol/L)²
= 1.35 x 10⁻³ x 7.29 x 10⁻⁶
= 9.8 x 10⁻⁹