Ideal Behavior of Water Solutions of Strong Electrolytes and Non-electrolytes at High Concentrations

Contrary to widely held beliefs, many concentrated aqueous solutions of strong electrolytes and nonelectrolytes are shown to behave ideally by calculating the activity of water (a _w) from vapor pressure data. The mole fraction of water (x _w) is equal to the water activity a _w(Raoult’s Law) when the mole fraction of water is calculated by accounting for water strongly bound to the solute, which is then not available to act as solvent. In this case x _w=(55.51−mH _T)/(55.51−mH _T+im), where m is the molality of the solute particles, i is the stoichiometric number of solute particles produced per mole of dissolved solute, and H _T is the thermodynamic hydration number H _T. Published reservations about previous work of this type are addressed. The values of H _T vary little over wide ranges of concentration and correlate with the Hofmeister series, the B coefficient of the Jones-Dole viscosity equation, and other properties of water. Activity coefficients of the bulk or “free” water remain at unity even at high concentrations.