For gas-phase first-order decomposition A(g) -> B(g) + C(g): initial pressure = $P_0$ (all A). At time t, if x amount decomposes: $P_A$ = $P_0$ - x, $P_B$ = x, $P_C$ = x. Total pressure $P_t$ = $P_0$ + x, so x = $P_t$ - $P_0$. $P_A$ = $P_0$ - ($P_t$ - $P_0$) = 2$P_0$ - $P_t$. k = $\frac{2.303}{t}$log($\frac{P_0}{2P_0 - P_t}$). At completion (t -> infinity): $P_{infinity}$ = 2$P_0$ (for A -> B + C). General: if A -> nB (n products from 1 reactant), $P_{infinity}$ = $nP_0$, and k = $\frac{2.303}{t}$log((n-1)\frac{P_0}{(n}$$P_0 - $P_t$)). This gaseous decomposition approach is frequently tested in JEE.