According to the Avogadro’s law equal volumes of all the gases at same temperature and pressure contain equal number of molecules.
This definition of Avogadro’s law can be explained from the Avogadro’s hypothesis. This hypothesis says that all the ideal gases at standard temperature and pressure occupy a volume of 22.414 dm3.
Let us understand this..
22.414 dm3 of an ideal gas at S.T.P = 1 mole of gas
22.414 dm3 of an ideal gas at S.T.P = 6.02 x 1023 molecules of gas
1 dm3 of an ideal gas at S.T.P = 6.02 x 1023 / 22.414
= 2.68 x 1022 molecules
This statement means that if we have only ideal gases at 0oC and 1 atm pressure at S.T.P having volume 1 dm3, these will have 2.68 x 1022 molecules.
If the conditions of temperature and pressure or both are equally changed for all such ideal gases then the number of molecules will be equal in 1 dm3 or equal volumes.
Let us understand the reason behind this with an example.
1 dm3 of H2 gas has a mass = 0.0899g
1 dm3 of O2 gas has a mass = 1.4384g
But when these are managed at S.T.P then the number of molecules will be equal in both.
Since the molecules of the gases are widely separated from each other so their different masses do not disturb the volumes.
One molecule is approximately 300 times than its own diameter from its neighbor at ordinary temperature and pressure.