Hard water often contains dissolved Ca 2+ and Mg 2+ ions. One way to soften wate

Question

Hard water often contains dissolved Ca2+ and Mg2+ ions. One way to soften water is to add phosphates. The phosphate ion forms insoluble precipitates with calcium and magnesium ions, removing them from solution. Suppose that a solution is 5.0×10-2 M in calcium chloride and 8.5×10-2 M in magnesium nitrate.

What mass of sodium phosphate would have to be added to 2.0 L of this solution to completely eliminate the hard water ions? Assume complete reaction. Express your answer using two significant figures.

Explanation / Answer

We have 2.0 L of solution.
Moles CaCl2 = 5.0 x 10^-2 x 2 = 0.10 = moles Ca2+
Moles Mg(NO3)2 = 8.5 x 10^-2 x 2 = 0.17 =moles Mg2+
3 Mg2+ + 2 PO43- >> Mg3(PO4)2
3 Ca2+ + 2 PO43- >> Ca3(PO4)2
Total moles = 0.10 + 0.17 = 0.27
Moles PO43- needed = 0.27 x 2 / 3 = 0.18
Molar mass Na3PO4 =164 g/mol
0.18 mol x 164 g/mol = 29.5 g Na3PO4

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