[sharbeldam]

Yes I checked that s is much smaller after I solved.
I dont mean to be close minded but I think the only logical way I can get this, is that if they put in the question " Assuming that Mg(OH)2 dissociates completely "

if that assumption is not made, we cant say that the moles of OH- is exactly double its moles, since simply the moles of OH- is " 2s " and the moles of Mg(OH)2 if it does NOT dissociate completely is XXX-s, if it did dissociate completely then it is " s ".

I really hope my point is logical a bit

[AWK]

K_sp is used only for ionic compounds.

[sharbeldam]

Just so it can be final, can you answer with a yes or no (I know you like to make people think more and not give direct answer AWK )

Despite it being equilibrium do we assume that the ionic compounds dissociate completely?

[AWK]

If not then the stability constant of complext is used. Eg Al(OH)₃ in water is simply ionic (very good approximation) - use K_sp, in 0.3 M NaOH mainly covalent Al(OH)₆^3- is present.

[Borek]

A   +   B <--------> 2C
2M      2M               0
-x       -x                 +2x
2-x      2-x               2x

2-x doesn't equal 2x in a usual equilbrium problem.

It doesn't. But first - -x and +2x come from a simple stoichiometry like the one you should follow here, second reaction you wrote is different from the one at hand.

Mg(OH)2 <-----> Mg+2  + 2OH-

That's not the reaction you are dealing with. Mg(OH)₂ is a solid, so it it is not present in a dissolved from in the solution:

Mg(OH)₂(s) Mg²⁺(aq) + 2OH^-(aq)