The first ionization of H2CO3 in aqueous solution is:

$$H_2CO_3_{(aq)} \rightleftharpoons HCO^-_{(aq)}+H^+_{(aq)}$$

The equilibrium constant for this reaction is:

$$K_a = \frac{[HCO^-][H^+]}{[H_2CO_3]} = 4.3 \times 10^{-7}$$