25. Ionisation constant of a weak base MOH, is given expression

${\mathrm{K}}_{\mathrm{b}}=\frac{\left[{\mathrm{M}}^{+}\right]\left[{\mathrm{OH}}^{-}\right]}{\left[\mathrm{MOH}\right]}$

Values of ionisation constant of some weak bases at a particular temperature are given below

$\underset{{\mathrm{K}}_{\mathbf{b}}}{\mathrm{Base}}$ $\underset{5.4×{10}^{-4}}{\mathrm{Dimethylamine}}$ $\underset{1.3×{10}^{-14}}{\mathrm{Urea}}$ $\underset{1.77×{10}^{-9}}{\mathrm{Pyridine}}$ $\underset{1.77×{10}^{-5}}{\mathrm{Ammonia}}$

Arrange the bases in decreasing order of the extent of their ionisation at equilibrium. Which of the above base is the strongest?

Given that, ionisation constant of a weak base MOH
${\mathrm{K}}_{\mathrm{b}}=\frac{\left[{\mathrm{M}}^{+}\right]\left[{\mathrm{OH}}^{-}\right]}{\left[\mathrm{MOH}\right]}$
Larger the ionisation constant (Kb) of a base, greater is its ionisation and stronger the base.
Hence, dimethyl amine is the strongest base.
$\mathrm{Kb}$ $\underset{5.4×{10}^{-4}}{\mathrm{Dimethylamine}}$>$\underset{1.77×{10}^{-5}}{\mathrm{Ammonia}}$>$\underset{1.77×{10}^{-9}}{\mathrm{Pyridine}}$>$\underset{1.3×{10}^{-14}}{\mathrm{Urea}}$