The reaction, A(g) → 2B(g) follows first order kinetics. Initial pressure of ‘A’ is 800 mm. After 20 minutes the total pressure observed is 1400 mm. Rate constant of the reaction is

$$\begin{gathered} \mathrm{A}\left(\mathrm{g}\right) \to 2\mathrm{B}\left(\mathrm{g}\right) \\ \mathrm{Initial} \mathrm{pressure} 800 \mathrm{mm} - \\ \mathrm{Pressure} \mathrm{after} 20 \mathrm{minutes} \left(800-\mathrm{p}\right)\mathrm{mm} 2\mathrm{p} \mathrm{mm} \\ \mathrm{After} 20 \mathrm{minutes} \\ 800-\mathrm{p}+2\mathrm{p}=1400 \\ \mathrm{p}=600 \mathrm{mm}; \\ \mathrm{K}=\frac{2.303}{\mathrm{t}} \log \frac{{\mathrm{p}}_{\mathrm{o}}}{{\mathrm{p}}_{\mathrm{t}}} \\ \mathrm{K}=\frac{2.303}{20} \log \frac{800}{\left(800-600\right)} \\ \mathrm{K}=\frac{2.303}{20} \log \frac{800}{200} {\min }^{-1}; \\ \mathrm{K}=\frac{2.303}{20}\times 2 \log 2 {\min }^{-1}; \\ \mathrm{K}=6.93\times {10}^{-2} {\min }^{-1}; \end{gathered}$$