The Relative Formula Mass of NaH2PO4 is 120 g/mol Therefore, the number of moles = 6.6/120 = 0.055 moles of NaH2PO4 which is also equal to the number of moles of H2PO4. [H2PO4-] = Number of moles oof H2PO4-/Volume of the solution in L = 0.055/ ( 355 ×10^-3) = 0.155 M Na2HPO4 undergoes complete dissociation as follows; Na2HPO4 (aq)= 2Na+ (aq) + HPO4^2- (aq)
1 mole of Na2HPO4 = 142 g/mol Therefore; number of moles = 8.0/142 = 0.0563 moles [HPO4 ^-2] is given by no of moles HPO4^2- /volume of the solution in L = 0.0563/(355×10^-3) = 0.1586 M Both H2PO4^2- and HPO4^2- are weak acids the undergoes partial dissociation Ka of H2PO4- = 6.20 × 10^-8 [H+] =Ka*([H2PO4-]/[HPO4(2-)] = (6.20 ×10^-8)×(0.155/0.1586) = 6.059 ×10^-8 M pH = - log[H+] = - log (6.059×10^-8) = 7.218
