Pure  $O_2$ diffuses through an aperture in 224 seconds, whereas mixture of  $O_2$ and another gas containing  $10\%O_2$ diffuses through the aperture in 336 sec under similar condition of pressure and temperature. Find the molar mass of the gas in g/mol.

The gaseous mixture contains $\begin{array}{l}{\text{A}}_{\left(\text{g}\right)}\to {\text{B}}_{\left(\text{g}\right)}{\text{+3C}}_{\left(\text{g}\right)}\\ 1\text{ 0 0}\\ 0.9\text{ 0.1 0.3}\end{array}$  and  $90\%\text{\hspace{0.17em}\hspace{0.17em}gas.}$
Average molecular weight of mixture  $\left(M_{mix}\right)=\frac{32\times 10+90\times M_{uk}}{100}\mathrm{........}\left(i\right)$
Now for diffusion of gaseous mixture and pure  $O_2$
  $\frac{r_{O_2}}{r_{mix}}=\sqrt{\frac{M_{mix}}{M_{O_2}}}\Rightarrow \frac{t_{mix}}{t_{O_2}}=\sqrt{\frac{M_{mix}}{32}}$
or   $\frac{336}{224}=\sqrt{\frac{M_{mix}}{32}}\mathrm{.........}\left(ii\right)$      
 $\therefore M_{mix}=72$
By (i) and (ii) mol weight of gas (M_uk) = 76.44