Final answer:
In the formula for the heat of vaporization, the amount of heat needed to vaporize the substance can be expressed as joules per gram (J/g) or joules per mole (J/mol). This energy is essential for overcoming intermolecular forces to convert a liquid to a gas at the boiling point.
Step-by-step explanation:
In the formula for the heat of vaporization, the quantity that represents the amount of heat needed to vaporize the substance is usually expressed in two units: joules per gram (J/g) or joules per mole (J/mol). The heat of vaporization (symbolized as ΔHvap in thermodynamics), is the energy required to transform a unit amount of a liquid into a gas at the substance's boiling point. This amount of heat is crucial because it helps to overcome the intermolecular forces present within a liquid, providing the energy necessary for the molecules to escape into the gaseous phase.
An example of the heat of vaporization can be seen with water, where the heat of vaporization of water is approximately 2,250 joules per gram. This means that each gram of water requires an energy input of approximately 2,250 J to vaporize. Similarly, the molar heat of vaporization refers to the energy absorbed by one mole of that substance as it converts from liquid to gas, and it is equal in magnitude but opposite in sign to the molar heat of condensation (the heat released when a gas condenses back to a liquid).