The volume of a gas isn't really a property of the gas at all! It is a property of the container in which the gas is placed. Gases expand or contract to evenly fill the container they occupy.2.3
As a result one can measure the volume occupied by a gas before the gas is put into the container or, similarly, after the gas is removed, as well as while the gas is present.
The volume is an extensive property.
Paul J. Gans
2003-02-19