**Molar enthalpy of vaporization** is the amount of energy needed to change one mole of a substance from the liquid phase to the gas phase at constant temperature and pressure. The usual unit is kilojoules per mole (kJ/mol).

Because energy is required to vaporize a liquid, molar enthalpy of vaporization has a positive sign. This indicates energy is absorbed by the system to get the molecules into the gas state.

### Molar Enthalpy of Vaporization Formula

The mathematical formula used to calculate molar enthalpy of vaporization is:

**q = n⋅ΔH _{vap}**

- q is the amount of absorbed heat
- n is the number of moles
- ΔH
_{vap }is the molar enthalpy change of vaporization

This equation is rearranged to give:

**ΔH _{vap} = q/n**