Autoprotolysis constant of ${\mathrm{NH}}_3$ is

If ${\mathrm{CO}}_2$ is allowed to escape from the following reaction at equilibrium

${\mathrm{CO}}_2 + {\mathrm{H}}_2\mathrm{O} + {\mathrm{H}}_2{\mathrm{CO}}_3 \iff 2{\mathrm{H}}^{+}+ + 2 {{\mathrm{HCO}}_3}^{-}$