PCl5(g)⟷PCl3(g)+Cl2(g)
80% 20% 20%
Assuming the pressure is 1 atm.
The density of the mixture will have an 80% contribution from PCl5
20% contribution from PCl3 and
20% contribution from Cl2
Density of PCl5 at 25oC=208g/mol22.4L×273K523K×0.80=3.87g/L
Density of PCl3at25oC=137g/mol22.4L×273K523K×0.20=0.63g/L
Density of Cl2at25oC=70.9g/mol22.4L×273K523K×0.20=0.33g/L
Density of mixture =3.87+0.63+0.33=4.83g/L