Question

The vapour density of a mixture containing NO2 and N2O4 is 38.3 at 27'C. Calculate moles of NO2 in 100 g of mixture.

Answer 1

Vapour desity concept

Weight of a unit volume of gas or vapor compared to (divided by)
the weight of an equal volume of air (or, sometimes, hydrogen)

vapour density = mass of n molecules of gas / mass of n molecules of hydrogen

vapour density = molar mass of gas / molar mass of H2

vapour density = molar mass of gas / 2.01


Vapour density of gas = molar mass /2.01

Molar mass of gas mixture = 38.3*2.01= 77

Total Moles / 100 gm gas = 100/77 =1.3 moles


Let n is NO2 moles
(1.3-n ) = N2O4 moles

Mol wt of NO2 = 46
Mol wt of N2O4 = 92

n*46+ (1.3-n)*92=100
n=0.42

Therefore the given 100gm gas has following composition

NO2 = 0.42 moles or 0.42*46= 19.32 gms
N2O4=0.88 moles or 0.88*92 = 19.342 gms