HEAT OF VAPORIZATION

It takes a certain amount of energy to change a sample of liquid to its gaseous state, assuming that the matter is of the sort that can exist in either of these two states. In the case of water, it takes 540 cal to convert 1 g of liquid at +100°C to 1 g of pure water vapor at +100°C. This quantity varies for different substances and is called the heat of vaporization for the substance.
In the reverse scenario, if 1 g of pure water vapor at +100°C condenses completely and becomes liquid water at +100°C, it gives up 540 cal of heat. The heat of vaporization is expressed in the same units as heat of fusion, that is, in calories per gram (cal/g). It also can be expressed in kilocalories per kilogram (kcal/kg) and will yield exactly the same numbers as the cal/g figures for all substances. When the substance is something other than water, then the boiling/condensation point of that substance must be substituted for +100°C.
Heat of vaporization, like heat of fusion, is sometimes expressed in calories per mole (cal/mol) rather than in cal/g. However, this is not the usual case. If the heat of vaporization (in calories per gram) is symbolized hv, the heat added or given up by a sample of matter (in calories) is h, and the mass of the sample (in grams) is m, then the following formula holds:
h_v = h/m
This is the same as the formula for heat of fusion, except that hv has been substituted for h_f.