a
Given number density of molecules of gas as a function of $r$ is
$n(r)=n_{0} e^{-a r^{4}}$
Total number of molecule $=\int_{0}^{\infty} n(r) \mathrm{d} V=\int_{0}^{\infty} n_{0} e^{-\alpha x^{4}} 4 \pi r^{2} d r$
Number of molecules is proportional to $\mathrm{n}_{0} \alpha^{-3 / 4}$