Air and water vapour mixtures

The atmosphere consists of a mixture of dry air and water vapour. Air is itself a mixture of several elemental gases, mainly oxygen and nitrogen, hut the proportions of these are consistent throughout the atmosphere and it is convenient to consider air as one gas. This has a molecular mass of 28.97 and the standard atmospheric pressure is 1013.25 mhar or 101 325 Pa. 

Water may he present in air in the liquid form, as rain or mist, or as a solid (snow, hail). However, in general amhient and indoor conditions the water present in the air will he in the vapour form, i.e. as superheated low-pressure steam. 

If air and water are present together in a confined space, a balance condition will he reached where the air has become saturated with water vapour. If the temperature of the mixture is known, then the pressure of the water vapour will be the pressure of steam at this temperature (see also Section 1.3) (Table 23.1). Dalton s Taw of partial pressures (see also Section 1.5) states that the total pressure of a mixture of gases is equal to the sum of the individual pressures of the constituent gases, taken at the same temperature and occupying the same volume. Since the water saturation vapour pressure will remain constant, depending on temperature and not on volume, this pressure can be obtained from steam tables as below. The partial pressure exerted by the dry air must therefore be the remainder. 

Total (standard) pressure = 1013.25 mbar Partial pressure of saturated vapour = 31.66 mbar 

This calculation of the proportions by partial pressure can be converted to proportions by weight, by multiplying each pressure by the molecular mass (Avogadro s hypothesis), to give  

---