# Avogadro's number and the mole

## The hypothesis of Avogadro

In 1811, Lorenzo Romano Amedeo Carlo Avogadro, Conte di Quaregna e Cerreto hypothesised that:

Equal volumes of different gases placed at the same temperature and pression contain the same number of molecules.

This hypothesis was only confirmed and accepted in 1856 !

## Avogadro's number

2g of hydrogen gas occupy under normal conditions of temperature and pressure (NTP: 0^{o} C and 1 atm) a volume of 22.4 L
The number of molecules in that volume and under these conditions is called Avogadro's number.
According to Avogadro this number must be the same for all gases under normal conditions !

Avogadro's number is the number of molecules present in 22.4 L gas under normal conditions of temperature and pressure ( 0^{o} C and 1 atm)

## The determination of Avogadro's number

The french physicist Jean Perrin succeeded to determine Avogadro's number:

$N$ $=$ $6.023\cdot 10^{23}$

## The definition of the mole

A mole (of particles) are $N$ $=$ $6.023\cdot 10^{23}$ particles