Please hep with this lab question?

Question

Ksp for calcium phosphat is 1 x 10-33. The Ksp for CaF2 is 4 x 10-11. Ka for HF is 7.1 x 10-4. Ka for HPO42- is 1.7 x 10-12. In the elimination test, CaF2 precipitates from acetic acid,not Ca3(PO4)2. Offer an explanation for this behavior.

Answer 1

First of all you should give us more info, like what exactly is the elimination test, concentrations, conditions and if you know the pH of the final solution or the final concentration of acetic acid.

A very rough explanation based on the limited info, would be that HF is a much stronger acid than HPO4-2, so in an acidic environment it manages to dissociate much better, giving the conjugate base or if you prefer the conjugate base F- is much weaker than PO4-3 so PO4-3 will steal away the H+ from the acetic acid. Thus it is possible that in the acidified medium you have most of the phosphate in the form of HPO4-2, and the concentration of PO4-3 is too low to lead to precipitation.
On the other hand, although CaF2 is much more soluble, most of F- will remain as such and thus its concentration might be high enough to promote precipitation.