[eddzzz]

Hey guys. I'm a little stuck on this little question. I've plotted a distribution diagram of phosphoric acid already and I have three values of pKa's.

If I add an 0.1M of Ca3(PO4)2 (calcium phosphate) to 0.1M of phosphoric acid what affect will this have on the distribution of the species. I am given the Ksp for Ca3(PO4)2 which is 1.3x10^26.

I have no idea what to do from this.

Any advice would be great. Cheers

Ed

[AWK]

Hint
3 different phosphates of calcium are known. CaHPO₄ is also poorly soluble.

[cakaro13]

Have you thought of Le Chatelier's principal? Increasing the conc. of PO4 will shift the equilibrium....with Ca being an inert spectator ion.

[AWK]

If I add an 0.1M of Ca3(PO4)2 (calcium phosphate)

You can add 0.1 mole solid salt (poorly soluble) or ~10^-5 M solution. After mixing you can expext reaction.

[Borek]

Increasing the conc. of PO4 will shift the equilibrium....with Ca being an inert spectator ion.

Ca²⁺ is neither inert nor spectator in the presence of phosphoric acid.

[eddzzz]

Cheers for all the replies.

Ok I know that the calcium phosphate (Ca3(PO4)2) is poorly soluble because of its low Ksp. What reaction would I expect. I've looked everywhere on triprotic acids and the effect of poorly soluble salts and can't find anything.

[AWK]

Depending on amount of phoshoric acid you can expect also CaHPO₄, Ca(H₂PO₄)₂

[eddzzz]

Still struggling with this. I presume I have to do some sort of calculation that includes the Ksp to see how the distribution is affected.  

[AWK]

If I add an 0.1M of Ca3(PO4)2 (calcium phosphate) to 0.1M of phosphoric acid
Ed

This is insufficient and incorrect information.
 0.1M of Ca3(PO4)2 impossible concentration - calcium phospate form at least solution  ~10^-6 M

0.1M of phosphoric acid - volume is missing

[eddzzz]

Hmm thanks. Cheers for your efforts. I'll skip tht question then