Question

$\underset{0.35\text{ M}}{CO_{3}^{2 -}} + \underset{0.35\text{ M}}{H^{+}} \rightleftharpoons \underset{0}{HCO_{3}^{-}};K = \frac{1}{4 \times 10^{- 11}}$

For 100% 0 0 0.35 M

For $HCO_{3}^{-}$ solution, $\left\lbrack H^{+} \right\rbrack = \sqrt{K_{a1},K_{a2}} = 1.4 \times 10^{- 8}\text{ M}$

Now, $K_{a2} = \frac{\left\lbrack H^{+} \right\rbrack\left\lbrack CO_{3}^{2 -} \right\rbrack}{\left\lbrack HCO_{3}^{-} \right\rbrack}$

$\therefore\left\lbrack CO_{3}^{2 -} \right\rbrack = \frac{4 \times 10^{- 11} \times 0.35}{1.4 \times 10^{- 8}} = 10^{- 3}\text{ M}$