When one gram of liquid water at its boiling point is changed into water vapor

Water needs 40.65 kJ/mole of energy.

Explanation:

One gram of liquid water changed into water vapor at its boiling point, energy is required called the heat of vaporization. Water has a heat of vaporization of 40.65 kJ/mol. Hydrogen bonding is present between water molecules so to break this hydrogen bond, heat energy is required. This heat energy is different for different liquids, water needs 40.65 kJ/mole of energy to break the bond and change into vapors.