Avogadro's Law, a fundamental principle of chemistry, states that equal volumes of all gases, under the same conditions of temperature and pressure, contain equal numbers of molecules. This concept is based on the molecular theory of ideal gases, which assumes that gas particles are in constant motion, have no volume, and do not attract or repel each other. As a result, the number of gas particles in a given volume can be directly related to its pressure, temperature, and volume according to the ideal gas equation, PV=nRT.