[blanny]

Hi, I've been working on this problem for a while and I still feel like I'm missing something.  I've done other problems that look similar to this earlier in the chapter, I just think this one is more involved somehow.

Here it is:

A saturated solution of Co(OH)2 has a pH of 12.93. Calculate

A. the equilibrium concentrations of Co2+ and OH-

B. the Ksp for Co(OH)2

Here's what I think:

Right off the bat I took the ph, converted it to pOH (14-pH) and found the [OH-] concentration by taking  10^(14-12.93). I got 0.085M. Next I wrote the equation.

Co(OH)2 ---> Co2+              + 2OH-

                         .5(0.085M)     0.085M

This just seemed way to easy for one of the last questions in the chapter.  Is there something I'm missing?

Next, in order to find the Ksp of Co(OH)2 i took [OH-]^2 x [Co+2]

or ((0.085)^2 ) x (.5(0.085)) = 3.1 x 10 ^ -4

Again, this just seems too easy.  I did notice that the equation says the solution is "saturated". Does this effect the calcuations? I would really appreciate some help.  Nobody in my study group could figure this out.

Thx

blanny

[Borek]

OH^- complexes Co²⁺, which makes calculations more difficult. No idea if you are expected to check complexes stability constants or not. Your result (K_so) is a way too high so you probably should account for complexation somehow.

[blanny]

Borek, is that in any way similar to Kf or K of formation. 

[Borek]

Borek, is that in any way similar to Kf or K of formation. 

IIRC stability constant and formation constant of the complex are the same, not sure though. Google both, or check wikipedia.