A solution containing 0.008 695 m KH2PO4 and 0.03043 m Na2HPO4 is a primary standard buffer with

Question:

A solution containing 0.008 695 m KH2PO4 and 0.03043 m Na2HPO4 is a primary standard buffer with a stated pH of 7.413 at 25°C. Calculate the pH of this solution by using the systematic treatment of equilibrium with activity coefficients from
(a) The Davies equation and
(b) The extended Debye-Hückel equation.