Question

A) A solution contains 1.49×10^-2 M potassium phosphate and 6.81×10^-3 M potassium bromide.
Solid silver acetate is added slowly to this mixture.

What is the concentration of bromide ion when phosphate ion begins to precipitate?

B) A solution contains 1.22×10^-2 M chromium(III) acetate and 1.22×10^-2 M aluminum nitrate.
Solid sodium phosphate is added slowly to this mixture.

What is the concentration of chromium(III) ion when aluminum ion begins to precipitate?

Answer 1

A) Ksp of Ag₃PO₄ = [Ag⁺]³*[PO₄^3-]

i.e. 8.89*10^-17 = [Ag⁺]³*1.49*10^-2

i.e. [Ag⁺]= 1.814*10^-5 M

Therefore, the solubility of silver phsophate = 6.05*10^-6 M

Hence, the concentration of bromide ion when phosphate ion begins to precipitate = 6.81*10^-3 - 6.05*10^-6 = 6.804*10^-3 M

B) Ksp of AlPO₄ = [Al³⁺]*[PO₄^3-]

i.e. 6.3*10^-19 = 1.22*10^-2*[PO₄^3-]

i.e. [PO₄^3-]= 5.164*10^-17 M

Therefore, the solubility of aluminium phsophate = 5.164*10^-17 M

Hence, the concentration of chromium(III) ion when aluminium ion begins to precipitate = 1.22*10^-2 - 5.164*10^-17 = 1.22*10^-2 M