Which law states that equal volumes of gases at equal pressures and temperatures contain equal numbers of molecules?

Avogadro's Law states that equal volumes of gases at the same temperature and pressure contain an equal number of molecules. This principle is critical in explaining the relationship between the volume of a gas and the amount of substance (in moles) it contains, allowing us to understand how gases behave under standard conditions. When temperature and pressure are held constant, if you have two equal volumes of different gases, Avogadro's Law assures us that they must consist of the same number of molecules.

This concept is foundational in the study of gas behavior and is instrumental in various applications, including stoichiometry in chemical reactions, where the behavior of gases is relevant. Understanding this law aids in calculations involving gas volumes and helps predict how gases will interact under various conditions.