At a wastewater treatment plant, FeCl3(s) is added to remove excess phosphate from the effluent. Assume the following reactions occur: FeCl3 ---> Fe3+ + 3Cl- FePO4 ---> Fe3+ + PO4 3␣ The equilibrium constant for the second reaction is Ksp 1⁄4. What concentration of Fe3+ is needed to maintain the phosphate concentration below the limit of 1 mg P/L?

Answer : The concentration of needed is,

Explanation :

First we have to calculate the mole of phosphate.

As we are given that, 1 mg P/L that means, 1 mg of phosphate present in 1 L of solution.

Molar mass of phosphate = 94.97 g/mole

Now we have to calculate the concentration of phosphate.

Now we have to calculate the concentration of .

The second equilibrium reaction is,

The solubility constant expression for this reaction is:

Given:

Thus, the concentration of needed is,

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