Hypochlorous acid ionizes as:
$\mathrm{HOCl} \left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{H}}^{+} \left(\mathrm{aq}\right)+{\mathrm{OCl}}^{-} \left(\mathrm{aq}\right)$
${\mathrm{OCl}}^{- }\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)\rightleftharpoons \mathrm{HOCl} \left(\mathrm{aq}\right)+{\mathrm{OH}}^{-} \left(\mathrm{aq}\right)$
${\mathrm{K}}_{\mathrm{a}}$ for this reaction at $25° \mathrm{C}$ is $3.0\times {10}^{-8} \left({\mathrm{K}}_{\mathrm{w}}=1.0\times {10}^{-14} \mathrm{at} 25° \mathrm{C}\right)$
Hence, ${\mathrm{K}}_{\mathrm{b}}$ for $\mathrm{HOCl}$ is: