${\mathrm{AB}}_2$ dissociates as : ${\mathrm{AB}}_{2( \mathrm{g})}\rightleftharpoons {\mathrm{AB}}_{\left(\mathrm{g}\right)}+{\mathrm{B}}_{\left(\mathrm{g}\right)}$When the intial pressure of ${\mathrm{AB}}_2$ is $500 \mathrm{mm}$$\mathrm{Hg}$, the total equilibrium pressure is $700 \mathrm{mm}$ Hg. Calculate equilibrium constant for the reaction, assuming that the volume of the system remains unchanged.