Given
#"Total pressure of moisture and He "P=1.20atm#
#"Partial pressure of sat.vapour at" 45^@C=(f)=71.88"torr"=71.88/760atm~~0.095atm#
#"Temperature "T=(45+273)=328K#
#"Pressure of He"=p_(He)=1.20-0.95=1.105atm#
#"Mass of He"=0.586g#
#n_(He)=(0.586g)/(4g"/mol")=0.1465mol#
Now by ideal gas equation
#p_(He)V_(He)=n_(He)RT#
#V_(He)=(0.1465molxx0.082LatmK^-1mol^-1xx328K)/(1.105atm)#
#~~3.57L#