Activity of solids in liquids

When we looked at the solubility of naphthalene in various solvents (Section 6.4), we found that in benzene the actual solubility was close to the truly ideal value, as predicted on the basis of Raoult s Law, but in both hexane and methanol it was considerably lower. The chemical potential of the solid solute (and hence its activity in the solid state) is the same in all cases the activity of the naphthalene in solution must also be identical, for at equilibrium 

Nevertheless the concentrations are different, and therefore, the activity coefficients must vary. In the ideal case 

Thus for hexane y — 0.30/0.12 = 2.5. The activity coefficient greater than unity indicates that a smaller quantity of naphthalene in solution is necessary for it to attain the chemical potential of solid naphthalene than if the solution were truly ideal. We could say that as the naphthalene does not like being in the solution its tendency to stay in the solid state is greater. 

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