A saturated water vapour is contained in a cylindrical vessel under a weightless piston at a temperature `t = 100 ^@C`. As a result of a slow introduction of the piston a small fraction of the vapour `Delta m = 0.70 g` gets condensed. What amount of work was perfomed over the gas ? The vapour is assumed to be ideal, the volume of the liquid is to be neglected.

Explanation

The condensation takes place at constant pressure and temperature and the work done is
`p Delta V`
where `Delta V` is the volume of the condensed vapour in the vapour phase. It is
`p Delta V = (Delta m)/(M) RT = 120.6 J`
where `M = 18 gm` is the molecular weight of water.