Charles' Law is a special case of the ideal gas law. It states that the volume of a fixed mass of a gas is directly proportional to the temperature. This law applies to ideal gases held at a constant pressure, where only the volume and temperature are allowed to change.

Charles' Law is expressed as:

V_{i}/T_{i} = V_{f}/T_{f}

where

V_{i} = initial volume

T_{i} = initial absolute temperature

V_{f} = final volume

T_{f} = final absolute temperature

It is extremely important to remember the temperatures are absolute temperatures measured in Kelvin, **NOT** °C or °F.

## Charles Law Example Problems

A gas occupies 221 cm^{3} at a temperature of 0 C and pressure of 760 mm Hg. What will its volume be at 100 C?

Since the pressure is constant and the mass of gas doesn't change, you know you can apply Charles' law. The temperatures are given in Celsius, so they must first be converted into absolute temperature (Kelvin) to apply the formula:

V_{1} = 221cm^{3}; T_{1} = 273K (0 + 273); T_{2} = 373K (100 + 273)

Now the values can be plugged into the formula to solve for final volume:

V_{i}/T_{i} = V_{f}/T_{f}

221cm^{3} / 273K = V_{f }/ 373K

Rearranging the equation to solve for final volume:

V_{f } = (221 cm^{3})(373K) / 273K

V_{f } = 302 cm^{3}