[zmasterflex]

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

[zmasterflex]

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.

[Borek]

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 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.