`[H^(o+)]` formed due to electrolyte oxidation `= 4mmol`
`{:(,Na_(2)HPO_(4),+,Na_(3)PO_(4),),("Intial mmoles",0.08xx100,,0.02xx100,),(,=8,,=2,):}`
`[H^(o+)]` will be used by `pO_(4)^(3-)` to give `H_(2)PO_(4)^(Theta)`
`{:(PO_(4)^(3-)+,2H^(o+)rarr,H_(2)PO_(4)^(Theta),,),(2,4,0,,),(0,0,2,,):}`
Thus, a new buffer containing `Na_(2)HPO_(4) (8mmol)` and `H_(2)PO_(4)^(Theta)` will remain in solution.
`:. pH = pK_(a_(H_(2)PO_(4)^(Theta)) + "log"([Na_(2)HPO_(4)])/([H_(2)PO_(4)^(Theta)])`
`= 7.2 + "log" (8)/(2) = 7.81`