Question

What is Avogadro's principle?

Answer 1

In Avogadro's principle is the law defined for the gases and it is the relation between the volume of the gas and the number of atoms or molecules of the gas, and the conditions for this is that the gases are at similar conditions of temperature and pressure.

Complete answer:
Daltons' atomic theory told us that when the elements combine with each other to form compounds then they do so in a simple atomic ratio and Gay-Lussac's law told us that when the gases will combine they will do so in volume. Berzelius correlated both the theories and gave a statement that under similar conditions of temperature and pressure equal volumes of gas will have an equal number of atoms.
Later Avogadro modified it and gave the statement as under similar conditions of temperature and pressure equal volume of gases will have an equal number of molecules. This was accepted for all gaseous reactions and it is widely known as Avogadro's Law also.
Let take an example to understand this:
The reaction of hydrogen and chlorine will give hydrogen chloride gas.
$Hydrogen+Chlorine\to Hydrogen\text{ }chloride\text{ }gas$
According to Avogadro’s law, n molecules of hydrogen and n molecules of chlorine gas will combine to form 2n molecules of hydrogen chloride gas. When we divide this with 2n, then we get that $\dfrac{1}{2}$ molecule of hydrogen and $\dfrac{1}{2}$ of chlorine gas will combine to form 1 molecule of hydrogen chloride gas. This obeys Avogadro's law.

Note:
This Avogadro’s law or Avogadro’s principle helps us to find the atomicity of the elementary gases and helps us to find the relation between the molecular mass and vapor density of the gas.