The molar heat of vaporization is the amount of heat needed to boil or condense 1 mole of a substance at its boiling point without changing temperature. For water, the molar heat of vaporization is 40.7 kilojoules per mole. This value can be used to calculate the energy required to boil a given mass of water by multiplying the molar heat of vaporization by the mass and dividing by the molar mass.

1. MOLAR HEAT OF
VAPORZATION

2. What is Molar Heat of
Vaporization?
The amount of heat necessary to boil (or condense)
1.00 mole of a substance at its boiling point.
Note the two factors:
1.It’s 1 mole of a substance
2.There is no temperature change.

3. What is Molar Heat of
Vaporization?
The units for the molar heat of vaporization are
kilojoules per mole (Kj/mol)
The molar heat of vaporization of water is 40.7
kJ/mol.
 q=(ΔHvap)(mass/molar mass)

4. Example 1:
49.5g of Water is being boiled boiled at its boiling
point of 100°C. How mmany kJ of energy is
required?

5. ACTIVITY
1. 80.1g of H2O exists as a gas at 100°C. How much energy
must be removed to turn the water into liquid at 100°C?
2. Calculate the heat of vaporization for water in J/g.
3. Using the heat of vaporization for water in J/g, calculate
the energy needed to boil 50.0g of water at its boiling
point of 100°C.