Science, Tech, Math › Science What Is Avogadro's Law? Definition and Example Share Flipboard Email Print Italian chemist Amedeo Avogadro proposed Avogadro's Law in 1811 to describe the behavior of gases at equal pressure. DEA/CHOMON/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated November 07, 2019 Avogadro's Law is the relation which states that at the same temperature and pressure, equal volumes of all gases contain the same number of molecules. The law was described by Italian chemist and physicist Amedeo Avogadro in 1811. Avogadro's Law Equation There are a few ways to write this gas law, which is a mathematical relation. It may be stated: k = V/n where k is a proportionality constant V is the volume of a gas, and n is the number of moles of a gas Avogadro's law also means the ideal gas constant is the same value for all gases, so: constant = p1V1/T1n1 = P2V2/T2n2 V1/n1 = V2/n2V1n2 = V2n1 where p is pressure of a gas, V is volume, T is temperature, and n is number of moles Implications of Avogadro's Law There are a few important consequences of the law being true. The molar volume of all ideal gases at 0°C and 1 atm pressure is 22.4 liters. If pressure and temperature of a gas are constant, when the amount of gas increases, the volume increases.If pressure and temperature of a gas are constant, when the amount of gas decreases, the volume decreases.You prove Avogadro's Law every time you blow up a balloon. Avogadro's Law Example Say you have 5.00 L of a gas which contains 0.965 mol of molecules. What will be the new volume of the gas if the quantity is increased to 1.80 mol, assuming pressure and temperature are held constant? Select the appropriate form of the law for the calculation. In this case, a good choice is: V1n2 = V2n1 (5.00 L)(1.80 mol) = (x)(0.965 mol) Rewriting to solve for x give you: x = (5.00 L)(1.80 mol) / (0.965 mol) x = 9.33 L Sources Avogadro, Amedeo (1810). "Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons." Journal de Physique. 73: 58–76.Clapeyron, Émile (1834). "Mémoire sur la puissance motrice de la chaleur." Journal de l'École Polytechnique. XIV: 153–190.