Let the total mass of the mixture be 100 g
Mass of H2=20100×100=20 g
No. of moles of H2=202=10 moles
Mass of O2=100−20=80 g
No. of moles of O2=8032=2.5 moles
Total number of moles of H2 and O2=10+2.5=12.5
Mole fraction of H2=1012.5=0.8
Total pressure of mixtures =1 bar
∴ Partial pressure of H2 is the mixture, pH2
=Mole fraction×Total pressure=0.8×1=0.8 bar.