Question

Calculate the molar solubility of calcium phosphate [Ca3(PO4)2 in a solution which is 0.10M in sodium phosphate (Na3PO4). Ksp for calcium phosphate is 1.1x10^-26

Answer 1

................Ca3(PO4)2 ==> 3Ca^2+ + 2[PO4]^3-
I...............solid.....................0...................0
C..............solid.....................3x.................2x
E................solid...................3x..................2x

Then Na3PO4 is completely (100%) ionized
.............Na3PO4==> 3Na^+ + [PO4]^3-
I............0.1 M..............0...............0
C...........-0.1...............0.3............0.1
E.............0.0............0.3...............0.1
Ksp = 1.1E-26 = (Ca^2+)^3[(PO4)^3-]^2
Ca^2+ is 3x from the Ca3(PO4)2
[PO4]^3- is 2x from the Ca3PO4)2 + 0.1 for the Na3PO4 for a total of 2x + 0.1
so Ksp = 1.1E-26 = (3x)^3(2x + 0.1)^2
Solve for x and that gives you the molar solubility of Ca3(PO4)2
Post your work if you get stuck. I think you can ignore the 2x in 2x+0.1. Try it at least and see if you can because it will simplify the math.