The aqueous phase solubility of lead sulfate is determined by mixing 10 g of PbSO4 with 3.78 L of distilled water. After allowing for the salt to dissolve, water is decanted and analyzed gravimetrically, which reveals that 50 mL of the water contains 2.2 mg of residue. The distilled water alone contains 0.3 mg of residue per 50 mL. What is the solubility product (pKsp) of PbSO4 in distilled water?