Question

Calculate the silver ion concentration, [Ag+] for the reaction [Ag(CN2)]- --> Ag+ + 2CN-. The formation constant is 1x10^21. The silver ion concentration was in a solution which was originally 5.89x10^-3 M in AgNO3 and 1.54x10^-1 M in KCN.

Answer 1

..................Ag^+ + 2CN^- ==> [Ag(CN)2]^-
Initial....0.00589......0.308..............0
change -0.00589...-0.0118...........0.00589
equilib....0...............0.296............0.00589
With such a large Kf of 1E21 for the complex then the reaction goes far to the right. So the easy way to see this is let's take that same equation and see it from the right to the left. Here is the ICE chart. Remember we are going from the right to the left.
------------------------------------------------------------------------------------------
..................Ag^+ + 2CN^- <== [Ag(CN)2]^-
..................0..........0.296............0.00598 M...........Initial
..................+x.........+2x................ -x........................Change
..................+x......0.296+2x.........0.00598-x.............equilibrium
Now plug those numbers into the expression for the formation constant.
Kf = 1E21 = [Ag(CN)2]^-/[Ag^+][CN^-]^2
Post your work if you get stuck. [Ag^+] will be a very small number of course.