A piston exerting a pressure of 1 atm rests on the surface of water at 100C . the pressure is reduced to smaller extent and as a result 9g of water evaporates and absorbs 20 kJ of heat. calculate
1.delta H
2. work done on water vapour
3. heat of vapourization
4. delta U

Dear Student,

 w=PVV=(Volume of vapour-volume of liquid)Volume of vapour >> Volume of liquid, therefore,V= Volume of vapourNow, PV=nRTor, V=nRTPTherefore, w=P×nRTP=nRTT=100°C=273+100=373 Kn=918=0.5 molTherefore, w=(0.5)(8.314)(373)=1550.56 J= 1.550 kJU=q+w= 20 kJ +1.55 kJ=21.55 kJH=U+PV =21.55+1.55=23.1 kJ Heat absorbed by 9 g of water=20 kJHeat absorbed by 18 g of water=20×189=40 kJTherefore, Hvap of water= 40 kJmol-1 

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