Raoult's Law, Water Activity and Moisture Availability in Solutions



    1. Author Caurie, formerly with the Dept. of Nutrition and Food Science, Univ. of Ghana, Legon, Ghana, is now with the Dept. of Food Technology, Federal Polytechnic, llaro Ogun State, Nigeria.
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A rationalization of Raoult's law has led to the conclusion that water activity (aw) is a joint solution property of vapor pressure, solute and solvent concentrations. It is shown from this that aw is not a measure of the absolute value of the mole fraction of water as indicated by Raoult's law but a measure of only a fraction of the mole fraction of water remaining free in solution available and unbound to solute molecules. The law is shown to overestimate this water activity (aw) at all dilutions by an amount equal to the product of the mole fraction of solute and the lowered relative vapor pressure the solute generates in solution.