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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: zmasterflex on March 28, 2014, 07:56:23 AM

Title: Dissociation Concept
Post by: zmasterflex on March 28, 2014, 07:56:23 AM
Concept question. Why do I sometimes break down Al(OH)3 into the format
27x^4 when solving a problem and sometimes just into [Al][OH]^3? I know that it has something to do with the difference between kinetics and thermodynamics but if someone can just explain A) what that means and B) how the breakdown relates to the meaning, it would be very helpful. Thanks
Title: Re: Dissociation Concept
Post by: zmasterflex on March 28, 2014, 08:02:53 AM
Let me just clarify with an example;
Calculate the pH of a solution necessary to just begin the precipitation of Mg(OH)2 when [Mg2+] = .001M. (Ksp for Mg(OH)2 = 1.2e-11)

I understand that we write out the equation then divide out the Mg to isolate the OH, what I don't understand is how we set it up;
Is it [OH]^2 = 1.2e-11/.001
Or is it [2OH]^2 = 1.2e-11/.001

Thanks.
Title: Re: Dissociation Concept
Post by: Borek on March 28, 2014, 08:27:01 AM
Quote from: zmasterflex on March 28, 2014, 08:02:53 AM
Is it [OH]^2 = 1.2e-11/.001

Yes.

The other approach is when we try to calculate concentrations in saturated solution.

Say, we have PbCl2 dissolving:

PbCl2 :lequil: Pb2+ + 2Cl-

and we know

Ksp=[Pb2+][Cl-]

How do we find how much PbCl2 dissolved? Let's say solubility is x mol/L. That  means saturated solution contains x mol/L of Pb2+, but - because of the stoichiometry - twice as much Cl-. So

[Pb2+] = x
[Cl-] = 2x

Now plug these into the Ksp and you will see where these strange coefficients come from.