Question

State and explain Avogadro's law.

Answer 1

1. In the year 1811, Avogadro made a distinction between atoms and molecules and thereby proposed Avogadro’s law.
2. Avogadro proposed that, “Equal volumes of all gases at the same temperature and pressure contain an equal number of molecules”.
   e.g. Hydrogen gas combines with oxygen gas to produce water vapour as follows:
   \[\ce{\underset{\text{[2 vol]}}{\underset{\text{[100 mL]}}{Hydrogen}_{(g)}} + \underset{\text{[1 vol]}}{\underset{\text{[50 mL]}}{Oxygen}_{(g)}}->\underset{\text{[2 vol]}}{\underset{\text{[100 mL]}}{Water}_{(g)}}}\]
   According to Avogadro’s law, if 1 volume contains n molecules, then 2n molecules of hydrogen combine with n molecules of oxygen to give 2n molecules of water, i.e., 2 molecules of hydrogen gas combine with 1 molecule of oxygen to give 2 molecules of water vapour as represented below:
   \[\ce{\underset{\text{[2 molecules]}}{\underset{\text{[2n molecules]}}{Hydrogen}_{(g)}} + \underset{\text{[1 molecule]}}{\underset{\text{[n molecules]}}{Oxygen}_{(g)}}->\underset{\text{[2 molecules]}}{\underset{\text{[2n molecules]}}{Water}_{(g)}}}\]