Chapter 17 Principles of Chemical Reactivity: Other Aspects of Aqueous Equilibria - Study Questions - Page 677a: 19

See the answer below.

$K_a=[H_3O^+][HPO_4^{2-}]/[H_2PO_4^-]$ Applying the -log to both sides $pKa=pH-log([HPO_4^{2-}]/[H_2PO_4^-])$ $[HPO_4^{2-}]/[H_2PO_4^-]=1.96$ Mix them with a ratio of 1.96 mol of hydrogen phosphate per mol of dihydrogen phosphate.