A mixture of hydrogen and oxygen has volume $500{\text{\hspace{0.17em}cm}}^3$, temperature 300K, pressure 400k Pa and mass 0.76g. The ratio of masses of oxygen to hydrogen will be 

From, ideal gas equation 
$PV=nRT\Rightarrow 400\times {10}^3\times 500\times {10}^{-6}=n\left(\frac{25}{3}\right)\left(300\right)$$\Rightarrow n=\frac{2}{25}$
Let  $n_1\&n_2$ be no. of moles of hydrogen & oxygen respectively 
Also,  $n_1+n_2=n$
Using   $n=\frac{m}{M}$    $\frac{m_1}{2}+\frac{m_2}{32}=\frac{2}{25}$  
and  $m_1+m_2=0.76gm$    $\frac{m_2}{m_1}=\frac{16}{3}$