Chemistry
posted by HELP. PLEASE!! on .
A 0.130mole quantity of CoCl2 is added to a liter of 1.20 M NH3 solution. What is the concentration of Co2 ions at equilibrium? Assume the formation constant of Co(NH3)62 is 5.0× 10^31.

There are a couple of ways to look at this. I'll show you how to do it with a good bit of reasoning. There is another way that I think makes it easier to explain. I can show you that way if you have trouble understanding this.
.........Co^2+ + 4NH3 ==> Co(NH3)4^2+
I......0.130......1.20.......0
C.....0.130....4*0.130....0.130
E.......0........0.68.......0.130
The ICE chart above is based on this.
With such a HUGE Kf for the complex ion, essentially ALL of the Co^2+ will be transformed to the complex. Therefore, the equilibrium value for the complex will be essentially 0.130, the Co^2+ at equilibrium will be ALMOST (but not quite) zero (although my ICE chart shows zero), and the NH3 left will be just 1.2 at the start  the amount used. The Kf is
5E31 = [Co(NH3)2^2+/(Co^2+)(NH3)^4
Substitute 0.130 for [Co(NH3)4^2+], substitute 0.68 for [NH3} and substitute x for [Co^2+] and solve for x. 
it's actually Co(NH3)6^2. but the whole calculation was correct!!! thank you SO much!

i got it wrong... i got 1.22x10^32. help!

what is it supposed to be?

what is the answer....

4.74x10^31 is the answer. NH3 from ice table is 1.2(6*.130)= [.42] not [.68]