Avogadro's law

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

The volume of one mole of any gas at STP is 22.4 litres. A mole of any substance contains 6.02x10²³ molecules – a number known as Avogadro’s number or Avogadro’s constant (N_A).

Dalton’s law of partial pressures

The pressure in a container is the sum of the partial pressures (P₁ and P₂) of the gases that occupy the container.