Avogadro's law

Avogadro’s law states that equal volumes of all gases, measured at the same temperature and pressure, contain the same number of molecules. Malay: hukum Avogadro · Chinese: 阿伏伽德罗定律.

The law tells you that, for gases, counting particles and measuring volume amount to the same thing as long as the temperature and pressure are held constant. This is the reason one mole of any gas has the same molar volume, and it is why the volumes of reacting gases follow the same simple ratio as the moles in a balanced equation. It holds because the actual size of the gas molecules is tiny compared with the empty space between them, so what fills the volume is really the number of particles rather than the particles themselves.

Example. At the same temperature and pressure, a 1 dm³ sample of hydrogen and a 1 dm³ sample of oxygen contain the same number of molecules, even though oxygen molecules are heavier. In a reaction where two volumes of hydrogen react with one volume of oxygen, the gas volumes are in the ratio 2:1, exactly matching the mole ratio from the balanced equation.

Confusion to avoid. Do not confuse Avogadro’s law with the Avogadro constant. The law is a statement about equal volumes of gases containing equal numbers of molecules; the constant is a fixed number of particles per mole. It is also important that the law applies only to gases and only when temperature and pressure are the same for the gases being compared.

In this chapter Avogadro’s law is the principle behind the molar volume and behind using gas volumes directly in stoichiometry. Because it lets you treat a volume ratio as a mole ratio, it links the ideas of the mole, volume and the balanced equation together, and it saves a great deal of work in gas calculations across SPM Chemistry.