(A) Heavy fertilizer use can lead to phosphate pollution in lakes, causing an explosion of plant growth, particularly algae. Excess algae deplete the lake of the oxygen necessary for other plant growth and animal life. A lake in the middle of a large farm was found to contain the phosphate ion in the concentration of 5.13 x 10^-4 M. The lake measured 300 m x 150 m x 5 m. One method to neutralize PO₄^3- is to add a calcium salt. What mass of Ca(NO₃)₂ must be added to lower the phosphate ion concentration to 1.00 x 10^-12 M? K_sp = 1.30 x 10^-32 for Ca₃(PO₄)₂.

(B) In a laboratory procedure you are instructed to mix 350.0 mL of 0.200 M AgNO₃(aq) and 250.0 mL of 0.240 M Na₂SO₄(aq). What is the precipitate that you observe? To this mixture you add 400.0 mL of 0.500 M Na₂S₂O₃, causing the precipitate to dissolve. What is the mass of the remaining precipitate?