Since, the external pressure is 1.0 atm, the gas pressure is also 1.0 atm as piston is movable. Out of this 1.0 atm partial pressure due to unknown compound is 0.68 atm.Therefore, partial pressure of He=1.00−0.68=0.32 atm.⇒ Volume =p(He)n(He)RT​=0.320.1×0.082×273​=7 L⇒ Volume of container = Volume of He.​