The law that relates pressure and temperature at a constant volume is Gay-Lussac's Law.

Here's how it works:

* Constant Volume: The volume of the gas remains unchanged.

* Direct Proportionality: As the temperature of the gas increases, the pressure also increases proportionally. This means that if you double the temperature (in Kelvin), you will double the pressure.

Mathematical Expression:

Gay-Lussac's Law can be expressed mathematically as:

P₁/T₁ = P₂/T₂

Where:

* P₁ is the initial pressure

* T₁ is the initial temperature (in Kelvin)

* P₂ is the final pressure

* T₂ is the final temperature (in Kelvin)

Important Note: Temperatures must always be expressed in Kelvin (K) when using this law.