The Ksp expression for the system PbCl2 (s) Pb+2 (aq) + 2 Cl - (aq) is:

The solubility expression for this system would be the mathematical products of the chemical products, divided by the mathematical products of the chemical reactants, with each component being raised to the power denoted by their respective coefficients (gross!). The only exception is that components present in the solid state are excluded from this calculation. So:

Ksp = (Cl-) ^2 (Pb2+)

Notice that Cl is raised to the power of two, due to the coefficient present in the balanced reaction, and that PbCl2 is excluded from the calculation due to it’s state.