What Is the Formula for Charles' Law?

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 cm³ 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:

V₁ = 221cm³; T₁ = 273K (0 + 273); T₂ = 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
221cm³ / 273K = V_f / 373K

Rearranging the equation to solve for final volume:

V_f  = (221 cm³)(373K) / 273K

V_f  = 302 cm³