[antoinetta]

I have to calculate the concentration, in mol/L, of a saturated aqueous solution of Ca5(PO4)_3OH (Ksp = 6.80×10-37) but I'm having trouble setting up the question.

Is this the proper dissociation equation?

Ca5(PO4)_3OH(s) + 4 H3O+(aq) ? 5 Ca^2+(s) + 5 H2O (l) + 3 HPO4^2-(aq)

So, the only ones that I have to take into consideration is this:

Ca5(PO4)_3OH(s) ? 5 Ca^2+(s) + 3 HPO4^2-(aq)

So the Ksp equation is:

(5s^5)(3s^3) = 6.80×10-37

S = 7.30*10^-6

Not sure if I did this correctly :S

[DevaDevil]

First off, next time please use the nice features for sub- [ sub]..[ /sub] and superscript [ sup][ /sup] (both without the space in the [], or use the feature in the bar above the posting box); this makes your post easier to read.

Now for the question:
Dissolution of Ca₅(PO₄)₃OH :
Ca₅(PO₄)₃OH (s) <--> 5 Ca²⁺ (aq) + 3 PO₄^3- (aq) + OH^- (aq)
Simple dissolution does not need H₃O⁺, and also it is not mentioned in the question that it the solution is made acidous.

K_sp = [Ca²⁺]⁵ * [PO₄^3-]³ * [OH^-]

The rest you should be able to figure out

(PS. this is assuming the dissolution is so low the pH of the solution hardly changes so the acid-base equilibria do not apply (or this is already included in the K_sp), else this gets more complicated)