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Hard water often contains dissolved Ca^2+ and Mg^2+ 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 5.5×10?2 M in calcium chloride and 9.5×10?2 M in magnesium nitrate.

What mass of sodium phosphate would have to be added to 1.5 L of this solution to completely eliminate the hard water ions? Assume complete reaction.

Need some help guys. Thanks!


First you gotta figure out which phosphate you're using so you have the correct mass. Because you are reacting with +2 Ions you'll be needing disodium phosphate (Na2HPO4 -- FM = 141.96g/mol)

Next, find the moles of your elemental calcium / magnesium being reacted with:

.055 mol/L Ca+2 x 1.5L = .0825. Now you have .0825 moles Ca2+ and it's a 1:1 mol to mol ratio between Ca and phosphate, so .0825 mol x 141.96 g/mol = 11.7117g Na2HPO4 required!

Using this formula find the next one on your own! (HINT: this reaction is also 1:1 ratio)