Question

Vapour density of air consider it as 80% n2 and 20% o2

Answer 1

Vapour density = molar mass, M/2 
To find the molar mass of air, use the relative abundance formula modified for molecules 
Molar mass of air mixture, M_a = M_N2 X   Frac_N2  +  M_O2  X  Frac_O2 
where 
M_N2  is molar mass of N₂= 28g/mol 
Frac_N2 is fractional abundance of N₂  =  80% or 0.8 
M_O2 is molar mass of O₂ = 32g/mol 
Frac_O2 is fractional abundance of O₂ = 20% or 0.2 
SUbstituting the values 
M_a  =  28 X 0.8 +  32 X 0.2 
M_a = 22.4 + 6.4 
M_a = 28.8g/mol 
Vapour density of air = M_a/2 
Vapour density of air = 28.8/2 
Vapour density of air = 14.4