Question

50mL each of 0.050F CaCl2 and 0.050F phosphoric acid were mixed; Ksp, CaHPO4=2.6E-7. Consider only the equilibria of the species given in the problem with the obvious ones in aqueous solutions.( For triprotic acid, H3PO4, Ka1=7.11E-3, Ka2=6.32E-8 and Ka3=7.1E-13).

i) Explain why CaHPO4 would precipitate when phosphoric acid and calcium chloride were mixed together.

ii) List all major species(ions,molecules,solids etc) expected to be present in the system.

iii) write the set of equations that can be used to solve for the concentrations of all the species in the system. No need to solve equations

Answer 1

(i) CaHPO₄ is not soluble and hence forms precipitate.
(ii) Ca²⁺, HPO₄^2- , Cl^-, H⁺, H₂PO₄^-, HPO₄^-, PO₄^-, CaCl₂, H₃PO₄, CaHPO₄, HCl
(iii) CaHPO₄ <------------> Ca²⁺ + HPO₄^2-
   2.6*10^-7 = [Ca²⁺] [HPO₄^2- ]
   H₃PO₄ -----------> H⁺ + H₂PO₄^-
   K_a1 = [H⁺] [ H₂PO₄^-] / [H₃PO₄ ] = 7.11*10^-3
     H₂PO₄^- --------------> H⁺ + HPO₄^-
   K_a2 = [ H⁺] [HPO₄^-] / [ H₂PO₄^-] = 6.32*10^-8
     HPO₄^- ----------> H⁺ + PO₄^-
   K_a3 = [H⁺] [ PO₄^-] / [ HPO₄^-]