The number of molecules in one mole of all gases is the same. The ratio of the Universal Gas Constant to Avogadro's number is called Boltzmann's constant. The collisions of the molecules with the walls of the enclosure are perfectly elastic so that principles of conservation of kinetic energy, total energy, and momentum all apply. Collisions between molecules are negligible compared with collisions with the enclosure walls. Attractive forces between the molecules are negligible.
Humidity is a measure of the amount of water vapor contained in the air.
Relative Humidity is the degree or percentage of saturation. It is the ratio of the actual water vapor content to the maximum amount that the air could contain if saturated.