Question

what is the principle reason for low solubility of nacl in benzene (c₆h₆)?
a) strong solvent - solvent interactions
b) strength of the covalent bond in nacl
c) weak solvation of na+ and cl - by c₆h₆
d) increased disorder due to mixing of solute and solvent

Explanation:

• Option a: Strong solvent - solvent interactions in benzene don't directly explain why NaCl is insoluble. Benzene has weak London dispersion forces, and this is not the main reason for NaCl's low solubility.
• Option b: NaCl has ionic bonds, not covalent bonds. So this option is incorrect.
• Option c: NaCl is an ionic compound, and for it to dissolve, the ions ($\ce{Na+}$ and $\ce{Cl-}$) need to be solvated by the solvent. Benzene is non - polar, and it can't effectively solvate the polar $\ce{Na+}$ and $\ce{Cl-}$ ions. This weak solvation leads to low solubility.
• Option d: Mixing solute and solvent generally increases disorder (entropy), but for ionic compounds dissolving in non - polar solvents like benzene, the energy required to separate the ions (lattice energy) is not compensated by the solvation energy. So increased disorder is not the reason for low solubility here.

Answer:

c) weak solvation of $\ce{Na+}$ and $\ce{Cl-}$ by $\ce{C6H6}$