The relationship between the volume of a gas to the number of molecules at constant temperature and pressure is known as Avogadro's law. It states that equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules. 22.4 litres of any gas at STP contains 6.023 x 1023 number of molecules irrespective of its nature. Therefore, the volume of a gas is directly proportional to the number of molecules N.
V 
N (at constant T and P)
The number of moles 'n' of a gas is also proportional to the number of molecules.
n = N/No
Where, No represents the Avogadro number and is constant. So,
n N,
V N or V n.
Thus, it can be concluded that the volume of gases at constant temperature and pressure is directly proportional to their number of moles.
