What is a Mole?

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:

Volume V
Mass M
Number of particles N
Amount of substance n
if it is a gas: "p·V - Value" p·V
m3, l, cm3
kg, g
kmol, mol
Pa·m3 = N·m, mbar·l

The characterisation of an amount of gas as p x V-value is mostly used in vacuum technology.