This was a question in a thermo exam. Is there any reason this can't be done by assuming p is constant and using W=p*deltaV? The evaporation of 1 kg of water at 100º C requires 540 kcal of heat (the "latent heat"). The vapour this process generates, at 1 atm, ocuppies 1.67 m^3. Assume the water vapour is an ideal gas. What fraction of the latent head corresponds to the work done against the atmosphere to accomodate the vapour?