The ratio of masses of oxygen and nitrogen in a particular gaseous mixture is 1:4. The ratio of number of their molecules is:

Let, Mass of O2 = w
Mass of N2 = 4w
Now, Molecules of O2 = w/32 × NA
Molecules of N2 = 4w/28 × NA
Ratio of molecules of O2 and N2 = (w/32 × NA) / (4w/28 × NA) = 7/32
Therefore, the ratio of the number of their molecules is 7:32