A solution contains 1.4 * 10^−5 mol of KBr and 0.14 mol of KCl per liter. AgNO3 is gradually added to this solution. Which forms first, solid AgBr or solid AgCl? Or do they co-precipitate?

Answer 1

I used Ksp for AgCl = 1.77E-10 and for AgBr = 5.35E-13. Your text may have different numbers.
Ksp AgCl = (Ag^+)(Cl^-). You know Cl^ from the problem. Calculate (Ag^+). I get approx 1E-9 M but you need a better answer than that.

Do the same for Ag^+ from AgBr. I get approx 4E-8 M. Again you need a better answer.
Now visualize your solution and you add the AgNO3 drop wise and with stirring. Which Ag^+ is reached first? Obriously the AgCl (the smaller number) will be reached first so AgCl will ppt first.

This is not part of the problem but some questions continue with, "What is the(Cl^-) when the first ppt of AgBr appears?" That is AgBr begins to ppt when (Ag^+) = approx 4E-8 so plug that number back in for Ag^+ to Ksp for AgCl and solve for Cl^-. I get about 5E-3. When enough AgNO3 has been added so that the Cl^- has been reduced from 0.14 to about 0.005 M, then both AgCl and AgBr will ppt together.