pKa of H2PO4- is = 7.21 According to Henderson\'s Equation , pH = pKa + log ( [salt] /[acid]) = pKa + log ( [Na2HPO4] / [ KH2PO4 ] ) = 7.21 + log ( 0.135 / 0.185 ) = 7.21 - 0.14 = 7.07 Solution pKa of H2PO4- is = 7.21 According to Henderson\'s Equation , pH = pKa + log ( [salt] /[acid]) = pKa + log ( [Na2HPO4] / [ KH2PO4 ] ) = 7.21 + log ( 0.135 / 0.185 ) = 7.21 - 0.14 = 7.07.