To an evacuated vessel with movable piston under external pressure of 1 atm, 0.1 mole of He and 1.0 mole of unknown compound (vapour pressure 0.68 atm at 0°C) are
introduced. Considering the ideal gas behaviour, the total volume (in litre) of the gases at 0°C is close to

Since, the external pressure is 1 .0 atm, the gas pressure is also 1.0 atm as piston is movable. Out of this 1 atm partial pressure due to unknown compound is 0.68 atm.

Therefore, partial pressure of He

= 1.00 - 0.68 = 0.32 atm

$\text{ Volume }=\frac{\mathrm{n}\left(\mathrm{He}\right)\mathrm{RT}}{\mathrm{p}\left(\mathrm{He}\right)}=\frac{0.1×0.082×273}{032}=7\mathrm{L}$