Autoionisation of liquid NH₃ occurs as
$2{\mathrm{NH}}_3\rightleftharpoons {\mathrm{NH}}_4^{+}+{\mathrm{NH}}_2^{-} \mathrm{with} {\mathrm{K}}_{{\mathrm{NH}}_3}=\left[{\mathrm{NH}}_4^{+}\right]\left[{\mathrm{NH}}_2^{-}\right]={10}^{-30}\text{ at }-{50}^{\circ }\mathrm{C}$
Number of amide ions, ${\mathrm{NH}}_2^{-}\text{, }$ present per mm³ of pure liquid NH₃ is