A solution is prepared by mixing 50.0 mL of 0.10 M sodium dihydrogen phosphate (NaH2PO4) with 25.0 mL of 0.20 M disodium hydrogen phosphate (Na2HPO4). The second dissociation constant of phosphoric acid corresponds to pKa2=7.20 at 25 °C. Assuming volumes are additive, what is the pH of the final solution?
a) 6.90
b) 7.07
c) 7.20
d) 7.60