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Answer» 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, Ma = MN2 X FracN2 + MO2 X FracO2 where 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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