Since it is not possible to calculate the weight of particles individually, a collection of such particles called mole is taken for all practical purposes. Avogadro discovered that under standard conditions of temperature and pressure, (1atm and 273K) a sample of gas occupies a volume of 22.4 L.
It was discovered that the number of atoms present in 12g of carbon is 6.023 x 1023 atoms. This is referred to as Avogadro number. A mole of a gas is the amount of a substance containing 6.023 x 1023 particles. Avogadro's number is the number of atoms present in C12 isotope = 12g of C. One mole of any gas at STP will have a volume of 22.4 L.