The amount of a gas is most conveniently (for chemists) measured by the number of moles of the gas present. If one has a gas mixture, one measures the number of moles of each component. While moles are the most convenient, they can't be measured directly. One has no moleometer. What we do, of course, is to weigh the gas and then compute the number of moles from that. We're going (usually) to ignore that minor complication.
The number of moles is an extensive property.
Paul J. Gans
2003-02-19