Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: blanny on December 09, 2006, 11:28:40 PM
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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
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OH- complexes Co2+, which makes calculations more difficult. No idea if you are expected to check complexes stability constants or not. Your result (Kso) is a way too high so you probably should account for complexation somehow.
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Borek, is that in any way similar to Kf or K of formation.
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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.