The half life period for catalytic decomposition of $$A{B_3}$$  at $$50\,mm\,Hg$$   is $$4\,hrs$$  and at $$100\,mm\,Hg$$   it is $$2\,hrs.$$  The order of reaction is

Solution :
$${t_{\frac{1}{2}}} \propto \frac{1}{{{{\left( p \right)}^{n - 1}}}}$$    where $$n$$ is the order of reaction
$$\eqalign{ & \frac{2}{4} = {\left( {\frac{{50}}{{100}}} \right)^{n - 1}}\,\,{\text{or}}\,\,\frac{1}{2} = {\left( {\frac{1}{2}} \right)^{n - 1}} \cr & n - 1 = 1;n = 2 \cr} $$