A gaseous mixture contains oxygen and another unknown gas in the molar ratio of 4:1 diffuses through a porous plug into 245 seconds, under similar conditions same volume of oxygen takes 220 seconds to diffuse. What is the molecular mass of the unknown gas?
I used the Graham's law of diffusion to find the molecular mass of mixture. i.e. $$\frac{r_\text{mix}}{r_{\ce{O2}}}=\frac{220}{245}=\sqrt{\frac{M_{\ce{O2}}}{M_\text{mix}}}$$ $$M_\text{mix}=39.6$$
But what's next? How do I calculate the $M_\text{gas}$?