Question

Ca₃(PO₄)₂ and CaSO₄ have certain solubility in water (they are not the same). Describe what happens to the solubility of each salt when each one is placed separately into 0.1000 M HCl. To obtain full credit, you must explain which salt shows the greatest change, if any, in solubility in 0.1000 M HCl. Assume you have an equal molar amounts of each salt and equal volumes of HCl.

H₃PO₄: K_a1 = 7.5 x10^-3 ; K_a2 = 6.2 x 10^-8 ; K_a3 = 4.8 x 10^-13

H₂SO₄: K = 1.2 x 10^-2

Answer 1

Lets assume that solibility of Calcium phosphate as S1 moles per liter of water and that of Calcium sulphate as S2 moles per liter of water

When these salts are separately dissolved in 0.1 M solution of HCl, both the salts dissociate to give their corresponding cations and anions.

The solubility of both these salts are affected. Let's see how :

When Calcium phosphate is dissolved in 0.1 M HCl solution, following are the different types of ions present in the solution :

Ca²⁺ , PO₄^3-, H⁺ , Cl^-

If all these ions remained independent of each other in the solution, the solubility of calcium phosphate would have remained unaffected. But here we see that the H⁺ and PO₄^3- ions interact ( they associate together to some extent) to form phosphoric acid. Thus, some of the ions coming from calcium sulphate go into the associated form of phosphoric acid, which decreases the ionic product of the solution.

This allows for more of the calcium phosphate salt to dissociate so that the phosphate ions which have moved to the associated H3PO4 are compensated. Thus we see that the presence of extra H+ ions due to 0.1M HCl increases the solubility of Calcium phosphate

Similar is the case of Calcium sulphate. Here the sulphate ions associate with H⁺ ions to form sulphuric acid. Thus its solubility also increases for the same reason

Now, which salt's solubility increases more depends upon which acid is more stable ( H2SO4 or H3PO4).

Since we see that K values of H₃PO₄ are very less in magnitude than those of H₂SO₄, it means that the tendency of H₃PO₄ to dissociate into ions is less as compared to H₂SO₄

Thus, more of the PO₄^3- ions get associated to form H₃PO₄, which means more of the calcium phosphate salt dissocaites to compensate for this loss

Thus, the solubility of calcium phosphate increases more as compared to that of calcium sulphate in 0.1 M HCl solution.