The $P^H$  of 0.1M weak mono acidic base is 9. The dissociation constant of the base is

$P^H=9,P^{OH}=5,\left[O{H}^{-}\right]=\sqrt{K_b.C}$

$\therefore K_b.C={\left[O{H}^{-}\right]}^2={10}^{-10}$

                  $\left(or\right)$

$K_a=\frac{{10}^{-10}}{0.1}$

       $={10}^{-9}$