Boyle's Law

For a constant amount of gas at a constant temperature, the product of the pressure and volume of the gas is a constant.

P₁V₁ = P₂V₂

Charles's Law

For a constant amount of gas at a constant pressure, the volume of the gas is directly proportional to the absolute temperature.

V₁/T₁ = V₂/T₂

Gay-Lussac’s Law

At a given temperature and pressure, equal volumes of gas contain equal numbers of moles. This is sometimes also known as Avogadro’s law.

P₁/T₁=P₂/T₂

Avogadro's hypothesis

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

Thus, 1 mole of any gas (i.e. 6.02 x 10²³ gas molecules) at 1 atmosphere pressure and 0°C occupies a volume of approximately 22.4 litres.

Each of these laws is a special case of a more general law. That general law is called the Ideal Gas Law.

P V = n R T

At standard temperature and pressure (commonly abbreviated as STP), the value of the temperature is 0°C (273 K) and the pressure is at 1 atmosphere (760 mmHg or 760 torrs or 1.01 x 10⁵ Nm^-2 [pascals] ).

This implies that at STP, 71.0 g of chlorine gas (one mole of chlorine molecules) will occupy a volume of 22.41 litres.

In addition, 44.0 g of carbon dioxide (one mole of carbon dioxide molecules) will also occupy a volume of 22.41 litres at STP.

Dalton’s Law

Dalton's Law of Partial Pressure states that the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture.

Graham’s Law of diffusion

The rate at which gases diffuse is inversely proportional to the square root of their densities. 

Henry's Law

Henry's Law states that at a given temperature, the amount of gas dissolved in a solute is directly proportional to the pressure of the gas above the substance.

Critical pressure

The pressure required to liquefy a vapour at its critical temperature.

Examples: 

 O₂ 50 bar
 N₂O 72 bar
 CO₂ 73 bar

Critical temperature

The temperature above which a vapour cannot be liquefied by any amount of pressure.  Above this temperature, the substance is a gas; below it, a vapour.

Examples: 

 O₂ -118°C
 N₂O 36.5°C
 CO₂ 31.0°C