Question

At a wastewater-treatment plant FeCl³(s) is added to remove excess phosphate from the effluent. Assume that the reactions that occur are:

FeCl₃(s) <> Fe³⁺ + 3Cl^-

FePO₄ <> Fe³⁺ + PO₄^3-

The equilibrium constant for the second reaction is 10-26.4. What concentration of Fe³⁺ would be needed to maintain the phosphate concentration below the limit of 1 mg/L of P? (Hint: Assume all the P is in the PO₄^3- form so we’re asking for 1 mg/L PO4 3- as P.) Answer: 6.88×10^-18 mg/L

Answer 1

The value of equilibrium constant will always have the units of mol/L hence we convert the concentration of P from mg/L to mol/L by dividing it's concentration by its molecular weight.

Then find by concentration of Fe3+ by dividing the equilibrium constant by the concentration of P in mol/L , to get concentration of Fe3+ in mol/L then convert it to mg/L by multiplying with the molecular weight of Fe3+