Avogadro's Law

Avogadro's Law is a hypothesis by Amedeo Avogadro on experimenting gas law. 

Avogadro's Law
Avogadro's Law


Avogadro's Law

Avogadro's Law is a hypothesis by Amedeo Avogadro on experimenting gas law. We see a balloon bursts when accessive air is blown into this. Our four-wheelers carry on moving with four air-filled tires. Only a puncture makes your journey painful. These two examples relate to gas law. Similar to them, there are several scientific areas where this law is found.


A hypothesis by the Italian scientist Amedeo Avogadro on experimenting gas law is popularly known as Avogadro’s Law or Avogadro’s Principle. In his law, Avogadro proves that when the temperature and pressure conditions remain constant, the number of moles and volume of the gas are directly proportional to a particular mass of an ideal gas. It means the same volume of gases contains a similar number of molecules at a specific temperature and pressure. He proved this in 1811. 


 To mathematically describe this law, we can write


V ∝ n 


or V/n = k




V is the volume of the gas;

n is the amount of substance of the gas (measured in moles);

k is a constant for a given temperature and pressure.


It can also be written as-


      V1n2 = V2n1


or   V1/n1 = V2/n2


For example, at a constant temperature and pressure; we have 3.00 L gas containing 0.965 mol of molecules. If the quantity is increased to 1.80, what will be the volume of the gas?




V1n2 = V2n1


(3.00 L) (1.80 mol) = (x) (0.965 mol)


Rewriting to solve for x give you:


x = (3.00 L) (1.80 mol) / (0.965 mol)


x = 5.595 L


Avogadro’s law has some important consequences. They are-


  • At 0°C and 1 atm pressure, the molar volume of all ideal gases is 22.4 liters.
  • In constant pressure and temperature, a gas’s amount increases if the volume increases.
  • In constant pressure and temperature, a gas’s amount decreases if the volume decreases. 


Avogadro was born into a distinguished family of Italian lawyers in 1776. As a family tradition, he also studied law and started practicing in Italy. But later he turned his attention to natural sciences. He started his studies in private in physics and mathematics in 1800. Electricity was the very first experiment by him which he conducted with his brother.  


He started teaching at a high school in Vercelli. He taught natural science there. Later, he continued most of his career as a chemist at the University of Turin and contributed much to the field of physical chemistry. 


Avogadro was working on chemistry in a period when it was about to be established as a separate part of science. Many scientists were constantly trying to develop chemistry with their researches. Many famous scientists like Dalton and Ge Lussac were the contemporaries of Avogadro. He took ideas from the founding of them in his research. 


‘New System of Chemical Philosophy’ by John Dalton was published in 1808. In this, he illustrated both the laws of definite proportions and multiple proportions. These laws were well accepted. According to Dalton, each element has atoms. The atoms have an atomic weight

Dalton proposed that the combination of atoms forms each element. The atoms are the key units in any chemical reaction. As he could not measure atomic weight, he made an incorrect assumption. It was, in the most common compound between two elements, there was one atom of each.

The French scientist, Gay-Lussac belonged that time too. His subject of research was the chemical reaction of gases. The result of his experiments was revolutionary in chemistry. That the ratios of volumes of the reacting gases were small integer numbers is what he found. It was a great discovery to find out atomic weights. Dalton also researched on Gay-Lussac's law. But as it was opposing his newly formed atomic theory, Dalton tried to discredit the work of the French chemist. 





Avogadro was influenced by the two scientists and started working on gas’s chemical reaction. 

In an experiment, Avogadro noticed surprising facts about gas density in 1809. He noticed that two volumes of hydrogen and one volume of oxygen forms two volumes of water. It was quite unexpected as he thought only one volume of water vapor would be formed. This instance indicates that oxygen consists of two atoms (he called it molecules).


He stated about three types of molecules in his writings; integral molecules, constituent molecules, and elementary molecules. Besides recognizing diatomic gaseous elementary molecules, he found that they can also exist in monatomic, triatomic, and tetratomic format. 


Avogadro wrote a Memoria (concise note) about this research. This memorial was published in De Lamétherie's Journal de Physique in 1811. It clarifies the difference between an atom and a molecule. His discovery proves that Dalton might have confused atoms’ and molecules' concepts. He proved that atoms (assumed by Dalton) were actually molecules. So, an atom of nitrogen or oxygen contains two molecules. And thus, two molecules of water are the combination of two molecules of hydrogen and one molecule of oxygen.    


He provided the exact molecular formula for water, ammonia, nitric and nitrous oxides, ammonia, hydrogen chloride, and carbon monoxide. Later he described the formulas for, carbon disulfide, carbon dioxide, hydrogen sulfide, and sulfur dioxide in 1814. 


The scientist was not well acquainted with the famous contemporary scientists in his early career. Like his finding opposed to the research of famous scientists, they rejected his law. He did not get proper recognition for his contribution although his researches were translated into English and French in his lifetime. 


It took half a century to realize that his works are worthy. On the basis of this of law, scientists developed the idea of a mole. Mole is the SI unit of the number of substances. It can be converted to grams. Avogadro’s principle shows that the molar volume of all gases contains the same number of molecules. It is in one mole and we call it Avogadro’s number although Avogadro did not invent this number. 


An Austrian high school teacher, Josef Loschmidt was the first one to estimate the number. He later joined as a professor at the University of Vienna. He used kinetic molecular theory in 1865. He did this to estimate how many particles are there in one cubic centimeter. It is known as Loschmidt constant and it is 2.6867773 x 1025 m-3.


Jean Baptiste Perrin, a French physicist was the first to use the term Avogadro’s number. He estimated Avogadro’s number in 1909 by working on Brownian motion. This motion says that the random movement of microscopic particles suspended in a liquid or gas. Various other techniques have been used to find out this number. 


The current figure for the Avogadro number is 6.022 x 1023. Mol^-1 is the unit for Avogadro’s number. To symbolize this number, we use N. The number is enormous. For example, a standard soft drink can’s Avogadro's number is able to cover the surface of the earth to a depth of over 200 miles. If you spread a similar number of popcorn kernels across the United States of America, a depth of over 9 miles will cover the whole country. If you want to count the atoms in one mole at a speed of 10 million per second, it will approximately take 2 billion years!


Avogadro did other researches as well. His papers on the masses of atoms, their gas densities, and their compounds were published in Journal de Physique in 1815. In 1821, while still working as a professor at the University of Turin, he published papers on the masses of atoms and the proportions in which they combine. He further carried on experimenting on the detail of the physics of matter. He published four volumes between 1837 to 1841.  


Although he did not receive due recognition in his lifetime, we now remember as one of the founders of atomic-molecular chemistry.