[fobbz]

What mass of Na₃PO₄ would you add to 80.0 mL of 0.200 M HCl to obtain a buffer with a pH of 7.75?

Okay so Na₃PO₄ will dissociate in solution and PO₄^3- will react with H₃O⁺.
 0.016 moles H₃O⁺ will form and react with H₂O.

PO₄^3- +  H₃O⁺ HPO₄^2- + H₂O

HPO₄^2- reacts with water, and forms H₂PO₄^2- and OH^-.

HPO₄^2- + H₂O  H₂PO₄^1- + OH^-

Since this is a buffer, henderson hasselbach can be used.  for the pKa, I am unsure what to use. If I use HPO₄^2- pKa I get a value for the mass of phosphate that is waay off.

I called x/94.97 the moles for phosphate, and 0.016 mol of hydronium. 0.016mols of weak acid HPO₄^2- will form. From here I put these into HH equation, but I get 6.788g for phosphate, while the answer says much less for Na₃PO₄.

I thought i understood all the reactions taking place, but apparently not.

[Borek]

HPO₄^2- reacts with water, and forms H₂PO₄^2- and OH^-.

HPO₄^2- + H₂O  H₂PO₄^1- + OH^-

What about HPO₄^2- reacting with H⁺?