An irreversible gas phase reaction $2 \textbf{P} \rightarrow 4 \textbf{Q} + \textbf{R}$ is conducted in an isothermal and isobaric batch reactor. Assume ideal gas behavior. The feed is an equimolar mixture of the reactant $\textbf{P}$ and an inert gas. After complete conversion of $\textbf{P}$, the fractional change in volume is ________ (round off to $2$ decimal places).