A major pollutant in waste water are phosphate ions (PO 4 3- (aq)), which can be

Question

A major pollutant in waste water are phosphate ions (PO43-(aq)), which can be removed through the addition of Mg2+(aq) ions. If 3.00*103 L of polluted water contains 2.2*10-3 mol/L PO43-(aq) ions and is treated with 120 mol of solid MgCl2 (which all dissolves):

a) Determine the concentration of PO43-(aq) ions after the treatment.

b) Determine the mass of magnesium phosphate which precipitates.

c) Suppose that the same waste water contains the same concentration of F-(aq) ions. Which solid would precipitate first, MgF2 or MgPO4 upon the addition upon gradual addition of MgCl2?

Answers:

a) [PO43-]= 1.4*10-10 M

b) ~ 870 g

c) No answer given

Explanation / Answer

a) initial moles of PO4^3- = 2.2 x 10^-3 M x 3 x 10^3 L = 6.6 mol

initial Mg2+ added = 120 mol

concentration [Mg2+] = 120/3 x 10^3 = 0.04 M

concentration of PO4^3- ion after treatment = sq.rt.(Ksp/[Mg2+]^3) = sq.rt.(1.0 x 10^-24/(0.04)^3) = 1.25 x 10^-10 M

b) mass of Mg3(PO4)2 precipitated out = (6.6/2) mol x 262.9 g/mol = 867.57 g

c) If the same water has also F-

Ksp for MgF2 = 7.4 x 10^-11

Ksp for Mg3((PO4)2 = 1.0 x 10^-24

As Ksp of Mg3(PO4)2 is lower than MgF2, this will precipitate out first from water.

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