The molar heat of vaporization is the energy needed to vaporize one mole of a liquid. The units are usually kilojoules per mole, or kJ/mol. Two possible equations can help you determine the molar heat of vaporization. To calculate the molar heat of vaporization, write down your given information, choose an equation that fits the circumstances, then solve the equation using the given pressure and temperature data.

Write down your given information. In order to calculate the molar heat of vaporization, you should write down the information that the problem provides. The problem will either provide two pressure and two temperature values, or the molar heat of sublimation, and the molar heat of fusion. The molar heat of sublimation is the energy needed to sublime one mole of a solid, and the molar heat of fusion is the energy needed to melt one mole of a solid.

Decide which equation to use. When calculating the molar heat of vaporization, you have to decide which equation you will use based on the given information. If the problem provides the two pressure and two temperature values, use the equation ln(P1/P2)=(Hvap/R)(T1-T2/T1xT2), where P1 and P2 are the pressure values; Hvap is the molar heat of vaporization; R is the gas constant; and T1 and T2 are the temperature values. If the problem provides the molar heat of sublimation and the molar heat of fusion, use the equation Hsub=Hfus+Hvap, where Hsub is the molar heat of sublimation and Hfus is the molar heat of fusion.

Solve the equation. If you are using the equation ln(P1/P2)=(Hvap/R)(T1-T2/T1xT2); the value of the gas constant, R, is 8.314 J/Kxmol. For example, if P1=402mmHg, P2=600mmHg, T1=200K, and T2=314K, then Hvap is equal to 1834 J/mol. You then divide your answer by 1,000, because there are 1,000 joules in 1 kilojoule. The answer then becomes 1.834 kJ/mol. If you are using the equation, Hsub=Hfus+Hvap, then you subtract the Hfus from the Hsub. For example, if Hsub=20 kJ/mol, and Hfus=13 kJ/mol, then Hvap=7 kJ/mol.