The data below show the mole fraction solubility of benzoic acid in two solvents at
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
The data below show the mole fraction solubility of benzoic acid in two solvents at 25∘C.
Solvent χ (solubility)
Water 1.9×10−3
Hexane 4.5×10−4
Students initially expect benzoic acid to be more soluble in hexane than in water because most of the molecule is non-polar. The observed data contradict this expectation.
(a) Identify the strongest intermolecular force operating between benzoic acid molecules and water, and explain how it arises from the molecular structure.
(b) Benzoic acid can dimerize through hydrogen bonding when dissolved in non-polar solvents. Explain how this self-association affects its apparent solubility in hexane.
(c) Using the concept of competition between solute–solute and solute–solvent interactions, justify the higher solubility of benzoic acid in water than in hexane.
(d) Predict how the solubility trend would change at pH>9 and provide a mechanistic explanation based on acid–base chemistry and intermolecular forces.