First order gas phase reaction\( \mathrm{A \rightarrow B + C} \)\( p_{i} \) = initial pressure of gas A, \( p_{t} \) = total pressure of the reaction mixture at time \( t \)Expression of rate constant \( (k) \) is
\( \frac{1}{t} \ln \frac{p_{i}}{2p_{i} - p_{t}} \)
\( \frac{1}{t} \ln \frac{2p_{i}}{p_{i} - p_{t}} \)
\( \frac{1}{t} \ln \frac{p_{i}}{3p_{i} - 2p_{t}} \)
\( \frac{1}{t} \ln \frac{3p_{i}}{4p_{i} - p_{t}} \)