At a wastewater-treatment plant FeC13(s) is added to remove excess phosphate from the effluent.

Assume that the reactions that occur are:

FeCl3(s )= Fe3+ + 3Cl - Fe3+ + PO4 3- = FePO4(s)

The equilibrium constant for the second reaction is 10-26.4. What concentration of Fe3+ (in the unit of mg/L) would needed to maintain the phosphate concentration below the limit of 1 mg of phosphorus per liter?

(Hint: 1 mg of P is not 1 mg of PO4.)