If equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, then the volume of a sample of gas must depend on the number of particles. For example, two molecules of hydrogen chloride gas occupy twice as much volume as one molecule of hydrogen chloride gas, and three molecules of hydrogen gas occupy three times as much volume as one molecule of hydrogen gas. This has been proved experimentally and is known as Avogadro's law.
Avogadro's law states that the volume of a gas at constant temperature and pressure is directly proportional to the number of molecules of the gas, n:
V = (a constant)(n)
At constant temperature and pressure, more gas molecules take up more space.
Because the volume of a gas is proportional to the number of molecules of the gas, volumes of gases can be used to count numbers of molecules of gases. Avogadro's law connects the properties of gases to the stoichiometry of chemical reactions that involve gases.