pH of acids, bases and salts

Exercise 13

    

Using the table of acid-base couples, calculate the $pH$ of a 0.64  $M$ potassium dihydrogenphosphate solution

$H_2PO_4^{-}$: ampholythe $pH$ $=$ $\frac{1}{2}pKa_1$ $+$ $\frac{1}{2}pKa_2$ = $\frac{1}{2}$ 2.12 + $\frac{1}{2}$7.2 = 4.66