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

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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= MN2 X   FracN2  +  MO2  X  FracO2 
MN2  is molar mass of N2= 28g/mol 
FracN2 is fractional abundance of N2  =  80% or 0.8 
MO2 is molar mass of O2 = 32g/mol 
FracO2 is fractional abundance of O2 = 20% or 0.2 
SUbstituting the values 
Ma  =  28 X 0.8 +  32 X 0.2 
Ma = 22.4 + 6.4 
Ma = 28.8g/mol 
Vapour density of air = Ma/2 
Vapour density of air = 28.8/2 
Vapour density of air = 14.4 

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