Sol. $\quad t_{1 / 2}=10$ minutes $$ \begin{aligned} & \left(P_A\right)_{30 \text { min. }}=\left(P_A\right)_0\left(\frac{1}{2}\right)^{30 / 10} \\ & \left(P_A\right)_{30 \text { min. }}=100 \mathrm{~mm} \mathrm{Hg} \\ & \begin{array}{c} \text { at } t=0 \quad \begin{array}{c} A(g) \\ 800 \mathrm{~mm} \end{array} \rightarrow \begin{array}{c} 2 B(g)_{n \sim}+C(g) \\ 0 \end{array} \\ \text { at } t=30 \quad 100 \mathrm{~mm} \quad 1400 \mathrm{~mm} \quad 700 \mathrm{~mm} \end{array} \end{aligned} $$ Total pressure after 30 minutes $=2200 \mathrm{~mm} \mathrm{Hg}$