c
${v_{av}} = \sqrt {\frac{{8RT}}{{\pi M}}} $ [$\because$ ${v_{rms}},\,\,R \to $ constant]
$\Rightarrow$ $T \propto M$
$\Rightarrow$ $\frac{{{T_{{H_2}}}}}{{{T_{{O_2}}}}} = \frac{{{M_{{H_2}}}}}{{{M_{{O_2}}}}}$Þ$\frac{{{T_{{H_2}}}}}{{(273 + 31)}} = \frac{2}{{32}}$
$\Rightarrow$ ${T_{{H_2}}} = 19\,K = - \,254^\circ C$