Determination of solute properties by vapor pressure measurements of the solvent in dilute solutions

Minor decreases in the vapor pressure of water in solution were accurately determined using a newly established method. The decreases were found to be linear with solute concentration in the range 0.02–0.1 molal. A slope for the decrease of vapor pressure for one molal solutions based on single particles was calculated and compared with the slope generally expected according to Raoult's law. The differences were found to be proportional to the charge and radius of the solute particles. An experimental rule was formulated. It provides a good fit for electrolytes, nonelectrolytes, and polyelectrolytes. The results are interpreted according to a standard state, P°i, for the vapor pressure of each solution at 10−3 molal. It is assumed that the solution obeys Raoult's law in the range 10−3−10−1 molal, substituting P°i for the vapor pressure of water. P°i is smaller than P° by about 0.2% for electrolyte and 1.5% for polyelectrolyte solutions.