Concept:

The molar heat of vaporization (\( \Delta H_{\text{vap}} \)) is a standard thermodynamic constant representing the energy required to vaporize one mole of a liquid at its boiling point under standard pressure.

Formula:

$$ \Delta H_{\text{vap}} = H_{\text{vapor}} - H_{\text{liquid}} $$

Solution:

• Water has an exceptionally high heat of vaporization due to its extensive hydrogen-bonded network, requiring massive energy to completely separate the molecules into a gas.



• The experimentally determined value for the molar heat of vaporization of water is strictly \( 40.7 \text{ kilojoules per mole (kJ/mol)} \).



• This high value is the reason sweating is such a highly efficient cooling mechanism for the human body.

Why other options are incorrect:

Options A and B use units that are thousands of times too small (cal and Joules). Option C uses kilocalories, but 40.7 kcal/mol equals approximately 170 kJ/mol, which is incorrect. The numeric value 40.7 corresponds specifically to kilojoules.