A 10L helium balloon at a pressure of two bar at 27C. Some O₂ is now inside in balloon and final volume is made 30L. the pressure also fell to 1 bar at similar temperature. the partial pressure of O₂ at end is?

Solution) For helium.

 n(He)=P(1)V(1)/RT.
for oxygen n(O)=P(2)V(2)/RT

partial pressure of O2= (mole fraction of O2).(Total pressure)

= [  P(2).V(2) / P(2).V(2) + P(1).V(1)  ].(1)
= 1.30 / 1.30 + 2.10
=30/30+20=3/5 bar
Hence 0.6 bar (ANS)