Solid ${\mathrm{NH}}_4\mathrm{I}$on rapid heating in a closed vessel at 357° C develops pressure of 300 mm Hg owing to partial decomposition ofNH4 I into NH3 and HI but .he pressure gradually increases further (when the excess solid residue remains in the vessel) owing to the dissociation of HI. Calculate the final pressure (in mm of Hg) developed at equilibrium. ${\mathrm{K}}_{\mathrm{p}}$ for HI dissociation is 2 at 357° C. $\left[\mathrm{HI}\rightleftharpoons \frac{1}{2}{\mathrm{H}}_2+\frac{1}{2}{\mathrm{I}}_2\right]$.