Which law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules?

Avogadro's Law is the principle that states equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules. This law emphasizes the relationship between the volume of a gas and the number of moles, indicating that for any gas, regardless of its type, the volume occupied will be proportional to the number of particles in that volume when temperature and pressure are constant.

This concept is significant in chemistry as it provides a way to connect the macroscopic properties of gases with the microscopic behavior of molecules. For example, if you have two different gases, such as oxygen and hydrogen, at the same temperature and pressure, if you take equal volumes of each, they will contain the same number of molecules, which illustrates the direct relationship outlined by Avogadro's Law.

This law is foundational for understanding gas behavior and plays a crucial role in stoichiometry, allowing chemists to predict the outcomes of reactions involving gases.