Can air at 120 degrees contain more water than at 100 degrees? And can you calculate that?

Answer

Air ‘contains’ no water vapour. The amount of water vapor that can occur at a certain temperature is independent of the presence of air.

But how much water vapor can you have at a certain temperature? If you place a quantity of water in a closed container, the water molecules will be able to escape the attraction of their neighbors with the greatest speed: they pass into the gas phase. As more and more molecules enter the gas phase, more and more will collide with the water surface and return to the liquid phase. At some point, as many molecules will evaporate as molecules will return to the liquid phase: an equilibrium has occurred. The pressure at which this equilibrium takes place is called the (equilibrium) vapor pressure. If the temperature is increased, the molecules gain a higher velocity and more will be able to transfer to the gas phase: the equilibrium vapor pressure rises.

Data about the vapor pressure as a function of the temperature can be found, for example, at:

http://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html