For their constitutive relationship for the fluids we assumed that vapour and dry air are perfect gas and so we have the equation of a perfect gas law, then we use Dalton's law, which says that the density of the mixture vapour and dry air is the same of the partial density and also the pressure of the mixture dry air plus vapour is the same of the partial pressure and here we have the molar mass. Finally we have the capillary pressure which is given by the difference of gas pressure and water pressure.

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