⇒States of Matter

Correct Option is (C)

Let the mass be ',$x$'.

${\mathrm{n}}_{{\mathrm{H}}_2}=\frac{\mathrm{xg}}{2\mathrm{g}{\mathrm{mol}}^{-1}}=\frac{\mathrm{x}}{2}$

$\begin{array}{rl}& {\mathrm{n}}_{\mathrm{He}}=\frac{\mathrm{xg}}{4\mathrm{g}{\mathrm{mol}}^{-1}}=\frac{\mathrm{x}}{4}\\ & {\mathrm{n}}_{\text{Total }}=\frac{3\mathrm{x}}{4}\end{array}$

Now,

$\begin{array}{rl}& x_{{\mathrm{H}}_2}=\frac{\mathrm{n}}{{\mathrm{n}}_{\text{Total }}}=\frac{\mathrm{x}/2}{3\mathrm{x}/4}=\frac{2}{3}\\ & x_{{\mathrm{H}}_e}=\frac{\mathrm{n}}{{\mathrm{n}}_{\text{Total }}}=\frac{\mathrm{x}/4}{3\mathrm{x}/4}=\frac{1}{3}\end{array}$

Now,

$\begin{array}{rl}& {\mathrm{P}}_{{\mathrm{H}}_2}=x_{{\mathrm{H}}_2}\times {\mathrm{P}}_{\text{Total }}=\frac{2}{3}\times \mathrm{P}\\ & {\mathrm{P}}_{\mathrm{He}}=x_{\mathrm{He}}\times {\mathrm{P}}_{\text{Total }}=\frac{1}{3}\times \mathrm{P}\\ & {\mathrm{P}}_{{\mathrm{H}}_2}:{\mathrm{P}}_{{\mathrm{H}}_e}=\frac{2/3\mathrm{P}}{1/3\mathrm{P}}=2:1\end{array}$