Answer
$N{O_2}^-(aq) + H_2O(l) \lt -- \gt HNO_2(aq) + OH^-(aq)$
$K_b = \frac{[HNO_2][OH^-]}{[N{O_2}^-]}$
Work Step by Step
The ionization equation should describe the transfer of a proton from a water molecule to the base, therefore, the base and the water are the reactants, and the protonated base and a hydroxide ion are the products.
The ionization constant expression is based on:
$K_b = \frac{[Products]}{[Reactants]}$
* The water doesn't appear, because it is the solvent, and its concentration doesn't matter.
