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What is Avogadro’s law?

Avogadro’s Law

The law of Avogadro also known by the name of Avogadro ‘s hypothesis or the principle of Avogadro ‘s law of gases which indicates that under the same conditions of pressure and temperature , the volume equal to possess all gases contain the same amount of molecules . The law is named after Amedeo Avogadro who, in 1811, hypothesized that two given samples of an ideal gas with the same volume and at the same temperature and pressure contain the samenumber of molecules ; and therefore, the number of molecules or atoms in a specific volume of ideal gas is independent of its size or the molar mass of the gas.

What is Avogadro’s law?

Avogadro’s law or Avogadro’s hypothesis is a gas law that is responsible for establishing a relationship between the volume and the amount of gas under pressure and over the temperatures that are constant .

What is Avogadro’s law?

Avogadro’s law states that, for a mass of an ideal gas , the volume of the gas and the number of moles are directly proportional if the temperature and pressure are constant . From the law of it can be deduced that for a given mass of an ideal gas, its volume and the number of molecules are directly proportional as long as the temperature and pressure are constant. The most significant consequence that Avogadro’s law has taught us is that the constant R for ideal gases has thesame value for all gases.


Amadeo Avogadro , an important Italian physicist , proposed in 1811 two different hypotheses , one of them said that the atoms of elemental gases were found together in molecules instead of being in separate atoms , as John Dalton had affirmed . The second hypothesis I had was that equal volumes of gases at constant pressure and temperature had the same number of molecules.

Avogadro’s hypothesis that it was related to the number of molecules that gases had was not accepted until 1858 , when the Italian chemist Stanislao Cannizzaro built a logical system based on the hypothesis.

Who Proposed Avogadro’s Law

The law was first proposed in 1811 by Amedeo Avogadro , a senior physics professor who worked at the University of Turin for many years, but his theory was not well accepted until after 1858 , when an Italian chemist, Stanislao Cannizzaro , built a logical chemistry system based on it.


Avogadro’s hypothesis or law can be stated as follows:

” Equal volumes of different gases contain the same number of particles , at the same pressure and temperature “


Mathematically it can be expressed with the following equation:

V / n = K

V = volume of the gas , generally expressed in liters.

n = amount of the substance measured in moles .

Also, from the so-called ideal gas law we have the following:

PV = nRT

P = gas pressure is usually expressed in atmospheres (atm), in mm of mercury (mmHg) or in Pascal (Pa).

At present, we know that the pressure and temperature that are constant , the same amount of gas has the same volume regardless of the chemical element that forms it. The volume (V) is directly proportional to the amount of gas particles (n). Therefore the formula of Avogadro’s law is the following :

1 / n 1 = V 2 / n 2

And therefore:

  • If you increase the amount of gas, you increase the volume.
  • If decreases the amount of gas decreases volume.


Avogadro’s hypothesis can be tested as follows:

At 0ºC you have a 500.0 milliliter flask to which you can add or remove gas, a balance that measures its mass and a manometer that measures pressure. When the “tare” button is pressed, the mass of the flask will have to be deducted from the measurements.

Gas is then introduced into the flask until a certain pressure is reached; the mass of said gas must be noted. This procedure is repeated with the other gases to check that: if the volume, pressure and temperature are constant in all cases there are the same amount of particles (although the masses are different).

Avogadro’s law applications

The gas or Avogadro law can be used to explain the mechanics by which pressure , temperature and volume are affected . For example, we can observe it in air conditioners, refrigerators and in the formation of clouds.


Avogadro’s law has been of great importance for the world of chemistry because through it it is possible to know the number of molecules that are contained in a mole .


Some examples of exercises already solved regarding Avogadro’s law are:

Example 1: 0.5 moles of a gas occupying 2 liters. Calculate what the new volume will be if 1 mole of gas is added at constant pressure and temperatures.

1 / n 1 = V 2 / n 2

1 = 2 liters

1 = 0.5 moles

2 = 0.5 + 1 = 1.5 moles

2 = V 1 n 2 / n 1 = 2 1.5 / 0.5 = 6 liters

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