A Mole (abbreviated to "mol") is a unit of quantity. The definition is:
The weight of each gas in a volume of 22.415 litres (at 0°C and 1,013 bar) is equal to the relative mass of one molecule of this gas.
Example: 1 mol Helium = 22.415 litre has a weight of 4 gm.
Units of quantity can be expressed in different units:
The characterisation of an amount of gas as p x V-value is mostly used in vacuum technology.