For ${\mathrm{SO}}_2{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}\to {\mathrm{SO}}_{2\left(\mathrm{g}\right)}+{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}$, Pressures of ${\mathrm{SO}}_2{\mathrm{Cl}}_2\text{ at }\mathrm{t}=0\text{ and }\mathrm{t}=20$ minutes respectively are 700mm & 350 mm. When log (P₀/p) is plotted against time (t), slope equals to