Question

What is the relation between vapour density and molecular weight of a gas?

Answer 1

P V = n R T = W/M R T
      W = weight of gas,  M = molecular weight, n = number of moles
      P =pressure, V = volume, R = universal gas constant
      T = temperature in K.     d = W/V = gas volume density


P/RT = d/M =>
      d/M = 1.013*10⁵ Pa / (8.314 J/K/mol * 273.15 K) = 44.6 units.
      volume density = 44.6 * Molecular mass      at STP

(Relative) vapour density = V. D.
         = mass of a volume of gas / mass of equal volume of H2 at same P & T.
         = mass of n molecules of gas / mass of n molecules of H2 at same P & T.
         = mass of 1 molecule of gas / mass of 1 molecule of Hydrogen
         = M/2

Relative molecular weight = mass of 1 molecule of gas / mass of 1 atom of H2
         = M / 1

Vapour Density = Relative molecular weight / 2