I'm not convinced at all. In fact, I believe the "answer" 4 is plain wrong.

4 is only the ratio of the mean molecules' speed. To compute a diffusion coefficient, it also needs a mean free path or a mean collision frequency, which depends on the molecule size.

http://en.wikipedia.org/wiki/Diffusion_coefficient#Gases

"sigma is the average collision diameter"

You maybe right sir, but i think in this question we are dealing with Graham's Law , my school friend solved it like this but i think he is wrong see this .

According to Graham's law

( rate CH4 / rate SO2 ) ^2 = MM SO2 / MM CH4

= 64 / 16 = 4

so rate CH4 / rate SO2 = square root √4 = 2

CH4 diffuses 2 x as fast as SO2

his answer is 2 :/ answer should be 4