The principle, which was formulated by the Italian chemist Amadeo Avogadro (1776-1856) in 1811, states: at the same temperature and pressure, the same number of molecules will be contained in equal volumes of gases, regardless of their chemical nature and physical properties. This number is a physical constant numerically equal to the number of molecules, atoms, electrons of ions or other particles contained in one mole. Later, the Avogadro hypothesis, confirmed by a large number of experiments, was considered for ideal gases to be one of the main laws that came into science under the name Avogadro's law, and its consequences are all based on the assertion that the mole of any gas, in the case of identical conditions, will occupy the same volume, called molar.
Amadeo Avogadro himself assumed that the physical constant is a very large quantity, but only a lot of independent methods, after the scientist’s death, made it possible to experimentally establish the number of atoms contained in 12 g (is an atomic unit of carbon mass) or in the molar volume of gas (at = 273.15 K and p = 101.32 kPa), equal to 22.41 liters. It is customary to designate a constant as NA or less often L. It is named after the scientist - the Avogadro number, and it is approximately 6.022 • 1023. This is the number of molecules of any gas in the volume of 22.41 l, it is the same for light gases (hydrogen), and for heavy gases (carbon dioxide). Avogadro's law can be mathematically expressed: V / n = VM, where:
- V is the volume of gas;
- n is the amount of substance, which is the ratio of the mass of a substance to its molar mass;
- VM is the constant of proportionality or molar volume.
Amadeo Avogadro belonged to a noble family living in northern Italy. He was born on 08/09/1776 in Turin. His father, Filippo Avogadro, was a member of the judiciary. A surname in a Venetian medieval dialect meant a lawyer or official who interacted with people. According to the traditions that existed at that time, positions and professions were inherited. Therefore, at the age of 20, Amadeo Avogadro received a degree, becoming a doctor of law (church). He began to study physics and mathematics at the age of 25. In his scientific activity he studied electrical phenomena and research in the field of electrochemistry. However, Avogadro entered the history of science, making a very important addition to atomic theory: he introduced the concept of the smallest particle of a substance (molecule) that can exist independently. This was important for explaining the simple volumetric relations between the gases that entered into the reaction, and the Avogadro law became of great importance for the development of science and was widely applied in practice.
But this did not happen right away. Avogadro's law has been recognized by some chemists decades later. The opponents of the Italian professor of physics were beaten by such famous and recognized scientific authorities as Berzelius, Dalton, Davy. Their misconceptions led to many years of controversy about the chemical formula of a water molecule, since there was an opinion that it should be written not HO, but HO or H2O2. And only Avogadro’s law helped establish the composition of water molecules and other simple and complex substances. Amadeo Avogadro claimed that the molecules of simple elements consist of two atoms: O2, H2, Cl2, N2. From which it followed that the reaction between hydrogen and chlorine, resulting in the formation of hydrogen chloride, can be written as: Cl2 + H2 → 2HCl. When one Cl2 molecule interacts with one H2 molecule, two HCl molecules form. The volume that HCl will occupy should be two times the volume of each of the components that entered into this reaction, that is, it must equal their total volume. Only since 1860, Avogadro’s law began to be actively applied, and the consequences of it made it possible to establish the true values of the atomic masses of some chemical elements.
One of the main conclusions made on its basis was the equation describing the state of an ideal gas: p • VM = R • T, where:
- VM is the molar volume;
- p is the gas pressure;
- T is the absolute temperature, K;
- R is the universal gas constant.
The combined gas law is also a consequence of the Avogadro law. With a constant mass of matter, it looks like (p • V) / T = n • R = const, and its recording form: (p1 • V1) / T1 = (p2 • V2) / T2 allows you to make calculations when a gas transitions from one state (indicated by index 1) to another (with index 2).
Avogadro’s law also made it possible to draw a second important conclusion, which opened the way for the experimental determination of the molecular masses of those substances that do not decompose upon transition to a gaseous state. M1 = M2 • D1, where:
- M1 is the molar mass for the first gas;
- M2 is the molar mass for the second gas;
- D1 is the relative density of the first gas, which is set by hydrogen or air (by hydrogen: D1 = M1 / 2, by air D1 = M1 / 29, where 2 and 29 are the molar masses of hydrogen and air, respectively).