A mixture of Ar (40) and N2 gases have a density of 1.40 g L-1 at STP. Hence, weight fraction of N2 approximately in the mixture is

p=nVRT=WmVRT=dRTm ∴m( molar mass of mixture )=dRTp=1.40×0.0821×2731 =31.38gmol−1  Let mixture contains x part N2 and (1−x) part Ar.  ∴ 31.38=m1x+m2(1−x)x+(1−x) ⇒ 31.38=28+40(1−x)1 ∴ x=0.72