A gaseous phase reaction ${\mathrm{A}}_2\to \mathrm{B}+0.5\mathrm{C}$ shows an increase in pressure from $100 \mathrm{mm}$ to $120 \mathrm{mm}$ in $5 \min$. Now, $-\mathrm{\Delta }\left[{\mathrm{A}}_2\right]/\mathrm{\Delta t}$ should be

$$\begin{gathered} {\mathrm{A}}_2( \mathrm{g})\to \mathrm{B}\left(\mathrm{g}\right)+\frac{1}{2}\mathrm{C}\left(\mathrm{g}\right) \\ 100 00\dots \dots \dots \text{initial} \\ \text{100-P}\mathrm{P}\mathrm{P}/2\cdots .......\mathrm{after} 5 \min \end{gathered}$$

After 5 minutes
Total pressure $\Rightarrow (100-\mathrm{P})+\mathrm{P}+\frac{\mathrm{P}}{2}=120 \mathrm{mm}$
$\frac{\mathrm{P}}{2}=20$
$\mathrm{P}=40 \mathrm{mm}$
So, rate of disappearance of ${\mathrm{A}}_2=\frac{40 \mathrm{mm}}{5 \min }$

$=8 \mathrm{mm}/$ minute