Avogadro’s Law and Avogadro’s Number

November 6, 2013

Avogadro's law and Avogadro's number

Avogadro’s law states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

Avogadro’s law in the mathematical form is as follows:

V/n = k

Here, V is the volume of the gas, n is the amount of substance in the gas and k is the proportionality constant.

Avogadro’s law is named after Amedeo Avogadro (1776 to 1856), an Italian scientist, along with the Avogadro’s number or constant 6.022 x 1023.  This was due to his notable contributions to molecular theory and for his formulation of the Avogadro’s law.

Two practical examples of the Avogadro’s law are as follows:

1)      Blowing and inflating a balloon:  The balloon increases in volume with the increase in the number of molecules of air inside the balloon and thus exemplifies Avogadro’s law in a simple manner.

2)      Rapid release of projectiles like bullets:  The sudden rapid release of a bullet as it is pushed out of its trigger during a shooting sports event shows Avogadro’s law in action.  The reaction of gunpowder behind a bullet as the trigger is released increases the gas particles behind the bullet with an increase in volume which quickly projects the bullet forward out of the gun.

You may have heard of Avogadro’s number which is 6.022 x 1023.  The Avogadro’s number 6.022 x 1023 is the number of molecules found in 1 gram-mole of a gas known as its molecular weight.  The volume occupied by 1 gram mole of a gas is the same for all gases and is around 22.4 liters at a standard temperature and pressure.  For example, the molecular weight of carbon dioxide is 44 and 1 gram mole of carbon dioxide will contain 6.022 x 1023 molecules.  The molecular weight of oxygen is 32.  How many molecules will 1 gram mole of oxygen contain?  Yes, 1 gram mole of oxygen occupying the same volume as 1 gram mole of carbon dioxide will contain the same number of molecules, that is, 6.022 x 1023 as per Avogadro’s law.   One of the foundation stones of studying modern chemistry and doing stoichiometry calculations is the use of the Avogadro number.

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