Let the total mass of the mixture be 100 g
Mass of H2 =

20

100

× 100 = 20 g , No. of moles of H2 =

20

2

= 10 moles
Mass of O2 = 100 – 20 = 80 g, No. of moles of O2 =

80

32

= 2.5 moles
Total number of moles of H2 and O2 = 10 + 2.5 = 12.5
Mole fraction of H2 =

10

12.5

=

4

5

= 0.8
Total pressure of mixture = 1 bar
∴ Partial pressure of H2 in the mixture, pH2 = Mole fraction × Total pressure
= 0.8 × 1 = 0.8 bar