Question

a mixture contains N2O4 and and NO2 in the ratio 2:1by volume. the vapour density of the mixture is?

Answer 1

vapour density N2O4 = molar mass N2O4/molar mass H2 = 92.011/2.016 = 45.64 
vapour density NO2 = molar mass NO2/molar mass H2 = 46.0055/2.016 = 22.82 

The ratio is 2:1 in favor N2O4; therefore, 

v.d. N2O4 x 2 = 91.28 
+ v.d. NO2 x 1 = 22.82 
sum is 114.1 
average is 114.1/3 = 38.033 which rounds to 38.03.