Question

The values of Ksp for AgBr and AgCl, are 7.7x10^-13 and 1.6 x 10^-10. A solution containing a mixture of 2.0x10^-2 M Br and 2.0 x 10^-2M Cl is a candidate for separating using selective precipitation. Solid AgNO3 is added, with a miniature spatula, and without changing the volume of solution. Silver nitrate is very solube.

b. At what concentration of Ag+ will AgCl first begin to precipitate?
I got 8.0x 10^-9 which is the correct answer, i just don't know how to get part c below
c.What will the residual concentration of Br- in solution be just as the condition in part b. reached?

thank you!

Answer 1

Ksp AgCl....(Ag^+)(Cl^-)...1.6E-10
-------- = ------------- = -------
Ksp AgBr....(Ag^+)(Br^-)...7.7E-13

(Cl^-)/(Br^-) = 207.79 (too many sig figures but you can round at the end).

Substitute the Cl^- value you have from part b and calculate Br^-