The law of Gay-Lussac explains the relationship existing between pressure and the absolute temperature for a given mass of gas at constant volume.

Gay-Lussac's law states that, at constant volume, the pressure exerted by a given mass of gas is directly proportional to the absolute temperature.

This directly proportional relationship can be written as: $P\propto T\mathrm{or}\frac{P}{T}=k$
where:
P is the pressure of the gas.
T is the temperature of the gas (measured in Kelvin).
k is a constant.

Because the formula is equal to a constant, Gay-Lussac's law can be rewritten as: $\frac{P_1}{T_1}=\frac{P_2}{T_2}$

The law was named after the French chemist Joseph Louis Gay-Lussac. This law only applies to ideal gases.

Example:
A gas in a tank has a pressure of 500 torr at 100 K is heated to 200 K. What is the pressure with no change in volume?

$\frac{P_1}{T_1}=\frac{P_2}{T_2}$

$P_2=P_{1}x\frac{T_2}{T_1}=500\mathrm{liters}x\frac{200K}{100K}=1000\mathrm{torr}$

Gay-Lussac's law is based upon absolute temperature. The absolute temperature is calculated by adding 273 to the temperature in Celsius scale. 10°C is equiavalent to 283 K.

$T_{}=10\mathrm{°C}+273=283K$