Find the mass of water vapour per cubic metre of air at temperature 300K and rekative humidity 50%. The saturation vapour pressure at 300K is 3.6kPa and the gas constant `R=8.3JK^(-1)mol^(-1)`.

Explanation

At 300K, the saturation vapour pressure=3.6kPa. Considering 1m^(3) of volume,
`pV=nRT=(m)/(M)RT`
where m=mass of vapour and M=molecular weight of water,
thus, `m=(MpV)/(RT)`
`=((18gmol^(-1))(3.6xx10^(3)Pa)(1m^(3)))/((8.3JK^(-1)mol^(-1))(300K))~~26g.`
As the relative humidity is 50%,the amount of vapour present is 1m^(3) is 26gxx0.50=13g.