Question

At what Ca2 solution containing 10 M PO3? Express the Ca concentration in mg/L Ca and in mg/L as CaCO3. Ignore aqueous complex formation 5. concentration will precipitation of Ca,(PO)2(s) (pKap 26.0) occur in a

Answer 1

Ca3(PO4)2 (s) <---> 3Ca2+ (aq) + 2PO4^3- (aq)

Ksp = [Ca2+]^3 [PO4^3-]^2    , where Ksp = 10^- pKsp = 10^-26

10^-26 = [Ca+]^2 ( 10^-6)^2

[Ca2+] = 10^-7 M     , per 1L we have Ca2+ moles = 10^-7 mol

Ca2+ mass = moles x atomic mass of Ca2+

= 10^-7 mol x 40g/mol = 4 x 10^-6 g = 4 x 10^-6 x 10^3 mg = 4 x 10^3 mg

Thus Ca2+ concentration = 4000 mg/L

CaCO3 moles = ca moles = 10^-7

CaCO3 mass = moles x molar mass of CaCO3 = 10^-7 mol x 100 g/mol = 10^-5 g = 10^-5 x 10^3 mg

= 0.01 g

Thus CaCO3 concentration needed = 0.01 mg/L