Problem: 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. A solution is 0.055 M in calcium chloride and 0.090 M in magnesium nitrate.What mass of sodium phosphate would you add to 2.0 L of this solution to completely eliminate the hard water ions? Assume complete reaction.

FREE Expert Solution

We first need to determine the moles of Ca2+ and Mg2+ present in the 2.0 L solution, and then we can calculate the moles of PO43– needed to react. Recall that molarity is given by:


For Ca2+: We're given 2.0 L of 0.055 M CaCl2 solution. Calcium chloride dissociates to Ca2+ and Cl:

CaCl2(aq) → Ca2+(aq) + 2 Cl(aq)

From this, we can see that 1 mole of CaCl2 forms 1 mole of Ca2+. The moles of Ca2+ present:

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Problem Details

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. A solution is 0.055 M in calcium chloride and 0.090 M in magnesium nitrate.

What mass of sodium phosphate would you add to 2.0 L of this solution to completely eliminate the hard water ions? Assume complete reaction.

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