Question

Calculate the amount of NaH2PO4 and Na2HPO4 needed to prepare 100 mL of a buffer with a pH = 8.25 so that the sum of concentrations of HPO4 and H2PO4 ions is 0.5 M.
Where do I start? :/

Answer 1

With the Henderson-Hasselbalch equation.
pH = pKa + log [(base)/(acid)]
You have pH and you can look up pKa. Solve for (base)/(acid) ratio. The second equation you want is
base + acid = 0.5 M
Solve the two equations simultaneously.