[_cheers]

Given that the stepwise dissociation constants for phosphoric acid are:
Ka1 = 7.5×10-3; Ka2 = 6.2×10-8; Ka3 = 4.8×10-13

To prepare 1.35 L of a buffer solution having an ionic strength of 0.090 and a pH of 7.65 would require:


(mass) of Na2HPO4(anhydrous) and


(mass) of NaH2PO4(anhydrous).

I'm not too sure where to start on this one, but i'm using Ka = 6.2x10-8 in my calcs?
thanks