Heat of Vaporization Example Problem

Heat of vaporization is the amount of heat energy required to change the state of a substance from a liquid into a vapor or gas. It is also known as enthalpy of vaporization, with units typically given in joules (J) or calories (cal).

Heat of Vaporization Problem

This sample problem demonstrates how to calculate the amount of energy required to turn a sample of water into steam:

What is the heat in joules required to convert 25 grams of water into steam? What is the heat in calories?
What you know: Heat of vaporization of water = 2257 J/g = 540 cal/g

Note: You won't be expected to know enthalpy or heat values; they will be given in a problem or can be looked up in a table.

How to Solve

You can solve this problem using either joules or calories for heat.

Part I:

Use the formula q = m·ΔH_v in which q = heat energy, m = mass, and ΔH_v = heat of vaporization.
q = (25 g)x(2257 J/g)
q = 56425 J
Part II:

q = m·ΔH_f
q = (25 g)x(540 cal/g)
q = 13500 cal

Answer

The amount of heat required to change 25 grams of water into steam is 56425 joules or 13500 calories.

A related example illustrates how to calculate the energy when water changes from solid ice into steam.