pH of acids, bases and salts

Exercise 13

    

Using the table of acid-base couples, calculate the $pH$ of a 0.784  $M$ potassium dihydrogencarbonate solution

$HPO_4^{2-}$: ampholythe $pH$ $=$ $\frac{1}{2}pKa_1$ $+$ $\frac{1}{2}pKa_2$ = $\frac{1}{2}$ 7.2 + $\frac{1}{2}$12.3 = 9.75