Gases have different volumes at different temperatures and pressures, as we know from the Ideal Gas Law , and the pressures and volumes of gases in a mixture will depend on the amount of water vapor in the mixture.
So it would be pointless to talk comparatively about volumes of gas unless we had a standard framework of temperature, pressure, and water satruation in which to work.
To this end, we have two standards that we'll use in this class:
STPD is more useful than BTPS for inorganic chemists, because body temperature doesn't really mean anything if you're not working with the body. STPD uses 0 degrees C, 760 mmHg of pressure, and assumes that the mixture is dry -- that is, that there is no water vapor .
That is: