What is Avogadro's Hypothesis?

You have learnt in the previous post that one mole of any substance contains 6.022×10²³ number of entities. Today we will try to find out how this number comes into picture.

Scientists needed to determine the weight of any element relative to a standard element, so they chose Carbon as standard element to determine the mass of atom and the number of atoms present in a mole. They choose Carbon ¹²C because its atomic weight is a whole number 12 atomic mass unit.

They measured the weight of one atom of Carbon spectroscopically. Then they used it to find the number of atoms present in the one mole i.e. 12g of carbon. Thus they got the number 6.022×10²³, and named it Avogadro Constant in honour of Avogadro

Avogadro proposed a hypothesis in which he said that equal volume of gases at the same temperature and pressure contains the same number of molecules. During his lifetime Avogadro's hypothesis was not well accepted but today it has become the base of mole concept.

Avogadro's hypothesis

This hypothesis is very simple to understand. Suppose you have to decorate a room for B’day party and for it you are filling air in balloons. If you want 4 balloons of equal size, then what you will do? Of course you fill them with equal amount of air. That means equal volume of air in the balloons contains equal number of molecules.

And if you want a few bigger balloons, then you will force more air to fill more air in the balloons. It’s obvious that larger volume contains more molecules. That means if you increase the volume, the number of molecules will increase. The number of molecules represents the number of moles. And mathematically you can say volume is directly proportional to the number of moles.

What is air? Air is the gaseous state of the molecules. Fascination towards air isn’t new, everyone has dreamed of flying in air, like birds. It motivates scientists to know more about it, that is why they continually experiment with it. Because of it today we are able to fly in air.

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