Question

Hard water often contains dissolved calcium ion and magnesium ions. One way to soften water is to add phosphate salts. 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 11.5 L of this solution to completely eliminate the hard water ions? (Assume complete reaction.)

Answer 1

[ca2+] = [cache] =5.5*10-2 m [mg2+] = [ng(NO3), 7= 9.5x102 m. - V = 11.5 l . Noget = 5.5X11.5810-2 moles - = 0.6325 moles & R est= 9.581105810-2 -1-0925 moles Suppose Moles of Na, po to be added to Completely eliminat ions= x moles 3 Ca2+ + 2Na₃PO4 ca3(PO4) + Gnat . 3 moles cazt requires 2 moles Na3PO4 . 0-63251 in 2x0.6325 " - = 0.4217 moles - Nazpo4 3 mg 2 + 2 Na₃PO4 mg (PO4 + 6 Nat" moles of Nazpo4 required = 2 x 1.0925 Hence - =0.7283 mder Total preso = 0.4217+0.7283 04 - 1015 moles wwazpon = 1.15X164 = 188.6 L