Question 13

Ok
Let us say total volume is V (mixture )
So V is also the volume of pure oxygen
Now rate of diffusion of mixture = V mL/ 245 sec
Rate of diffusion of pure oxygen = V mL/ 220sec
Total moles in the mixture are 5 if we take 4 mol of Oxygen gas and one mole of unknown gas.
Let us say molar mass of unknown gas as x
so molar mass of the mixture will be   (4 O2 + x)/5 = (128+x)/5

Ratio of rate of diffusion  of mixture and pure oxygen gas
you will get
((128+x))/(5(32))  =  ( 245/220)^2
solve it for x

Maam I did the same way and got same answer, 70 something but the ans is 133.Don't know what we are doing wrong.
Help