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I know that the common ion effect will decrease the solubility of $\ce{Ca3(PO4)2}$ since Phosphoric acid has a same anion. However, phosphoric acid is a weak acid so I am really having a hard time even writing a net chemical equation.

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    $\begingroup$ You should elaborate what you already did and SE members will help you where you have hit a snag. $\endgroup$ – Poutnik Jul 3 at 13:44
  • $\begingroup$ You should consider the opposite effect, as phosphoric acid presence eliminates most of phosphate and hydrogenphosphate ions by association with hydronium ions. $$\ce{Ca3(PO4)2 + 4 H3PO4 -> 3 Ca(H2PO4)2}$$, resp. $$\ce{Ca3(PO4)2 + 4 H+ -> 3 Ca^2+ + 2 H2PO4-}$$ Solubility of calcium dihydrogenphosphate is 2g/100 mL. $\endgroup$ – Poutnik Jul 3 at 20:18
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I'm quentin, I have your answer on the solubility of Ca3(PO4)2 :

Ca3(PO4)2 = 3 Ca^2+ + 2 PO4^3-

Ks = [Ca2+]3 [PO4^3-]2[Ca^2] = 3s and [PO4^2-] = 2s

Ks = s^5.3^3.2^2 = 108.s^5

10^-26 = 108.s^5 so

s = (10-26/108)1/5 = 2,5.10-6 mol.L^-1

the solubility of tricalcium phosphate is too low

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