Raoult's law revisited: accurately predicting equilibrium relative   humidity points for humidity control experiments
  The equilibrium relative humidity values for a number of the most commonly used precipitants in biological macromolecule crystallisation have been measured using a new humidity control device. A simple argument in statistical mechanics demonstrates that the saturated vapour pressure of a solvent is proportional to its mole fraction in an ideal solution (Raoult's Law). The same argument can be extended to the case where solvent and solute molecules are of different size. 
Michael G. Bowler, David R. Bowler, and Matthew W. Bowler
cond-mat.soft physics.bio-ph