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). This example problem demonstrates how to calculate the amount of energy required to turn a sample of water to steam.

### Heat of Vaporization Problem

What is the heat in joules required to convert 25 grams of water into steam? What is the heat in calories?

Useful information: 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.

### Solution

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

**Part I**

Use the formula q = m·ΔH_{v}

where

q = heat energy

m = mass

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.