A student has studied the decomposition of a gas AB_3 at 25^° C. He obtained the following data. The order of the reaction is

A student has studied the decomposition of a gas $\displaystyle \mathrm{AB}_3$ at $\displaystyle 25^{\circ} \mathrm{C}$. He obtained the following data.

p (mm Hg)	$\displaystyle 50$	$\displaystyle 100$	$\displaystyle 200$	$\displaystyle 400$
relative $\displaystyle \mathrm{t}_{1 / 2}(\mathrm{~s})$	$\displaystyle 4$	$\displaystyle 2$	$\displaystyle 1$	$\displaystyle 0.5$

The order of the reaction is