On a certain humid day, the mole fraction of the water vapour in air at 25∘C is 0.0287. If total pressure of air is 0.977 bar, calculate the partial pressure of water vapour in air and relative humidity if the vapour pressure of water at 25∘C is 0.0313 bar. The partial pressure of water vapour (PH2O) and relative humidity (RH) respectively are:
Here, mole fraction of H2O(XH2O)=0.0287,Ptotal = 0.977 bar . Therefore,
PH2O=XH2O×Ptotal=0.0287×0.977 bar = 0.028 bar.
% Relative humidity = PH2O in airVapour Pressure of H2O×100=0.0280.313×100 = 8.94%