Straight forward question. If atoms forming a gas float far from each other, how can we assign a volume to it? if it is in a container, perhaps we'd say the atoms, or molecules, reach every corner and thus its volume is the container's, but what about when it's in the air (think vacuum)? Do we consider a closed surface that contains the outermost atoms and the volume it encloses is the gas's?
Because gas is compressible, you must specify its pressure along with the volume it happens to occupy in order to properly define its state. And since heating a gas causes its pressure to increase, a complete description of any gas will necessarily include calling out its temperature as well.
Since more gas atoms in a fixed volume will exert more pressure on the thing containing them, a complete account of the state of a parcel of gas also includes a count of how many atoms of it there are inside that container.
This is summarized in an equation called the ideal gas law:
pressure x volume = number of atoms x a constant x temperature.