Chemistry
posted by Angela .
A hard water sample contains 121 mg of CaCO3 per liter of water. Calculate the mass of Na3Po4 needed to remove all Ca^2+ ions from 2.50 L of the water sample?

Ca3(PO4)2 ==> 3Ca^2+ + 2PO4^3
Ksp Ca3(PO4)2 = ?. Look that up in your text.
Ksp = (Ca^2+)^3(PO4^3)^2
(Ca^2+) = 0.121/molar mass CaCO3.
Substitute into Ksp expression, solve for (PO4^3) (note this will be in mols/L), multiply by 2.5 to convert to 2.5 L, then grams Na3PO4 = mols PO4^3 x molar mass Na3PO4. 
I don't understand what u mean by substitute into ksp expression & solve for PO4^3?