Solubility equilbrium

Question

If NaCl is added to a solution containing 0.01 Μ Cu+,0.01 Μ Ag+ and 0.01 Μ Au3+ ions;
a. Which compound will precipitate first?
b. When AuCl3 starts to precipitate, find [Ag+].
c. When Cu2Cl2 starts to precipitate, calculate the [Ag+] and [Au3+].

(Ksp for AuCl3 = 2.7 . 10^–25)
(Ksp for Cu2Cl2 = 1 . 10^–6)
(Ksp for AgCl = 1.6 . 10^–10)

Answer 1

a.)  Looking at the Ksp values, you see that AuCl₃ is the least soluble so it will precipitate first. This will be followed by precipitation of AgCl and finally by Cu₂Cl₂.  

 

b.)  AuCl₃ ==> Au³⁺ + 3Cl^- and Ksp = [Au³⁺][Cl^-]³

2.7x10^-25 = (x)(3x)³ = 9x⁴

x = [Au³⁺] = 4.16x10^-7 M (check my math)

[Cl^-] = 3 x 4.16x10^-7 M = 1.25x10^-6 M

This is the [Cl^-] at the point where AuCl₃ begins to precipitate, so use this value in the Ksp expression for AgCl.

Ksp AgCl = [Ag⁺][Cl^-] = 1.6x10^-10 = [Ag⁺][1.25x10^-6]

[Ag⁺] = 1.28x10^{-4 M}

 

c.)  For this, do the same as for (b) but use [Cl-] determined after ppt of AgCl.