At $-20^{\circ} \mathrm{C}$ and 1 atm pressure, a cylinder is filled with equal number of $\mathrm{H}_2 . \mathrm{I}_2$ and HI molecules for the reaction $\mathrm{H}_2(\mathrm{~g})+\mathrm{I}_2(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{g})$, the $\mathrm{K}_{\mathrm{p}}$ for the process is $\mathrm{x} \times 10^{-1} \cdot \mathrm{x}=$
[Given : $\mathrm{R}=0.082 \mathrm{~L} \mathrm{~atm} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ ]