What happens to freezing point of benzene when small quantity of napthalene is added to benzene?

Solution

Chemistry ma’am · 4 steps

Naphthalene dissolves in benzene, and any dissolved solute makes the liquid harder to freeze.

1. Solute disturbs freezing. Naphthalene is a non-volatile solid that dissolves in benzene. When the solution freezes, only pure benzene leaves as a solid, and the dissolved naphthalene particles get in the way of the molecules packing into a crystal.
2. Lower vapour pressure. The solute also lowers the vapour pressure of the liquid. So the liquid's vapour pressure meets that of solid benzene only at a lower temperature, and that meeting point is the freezing point.

   Δ T_f = K_f m

3. Size of the drop. Δ T_f is the fall in freezing point, K_f is a constant of benzene (5.12 K kg mol⁻¹) and m is the molality of naphthalene. Any m > 0 gives a positive Δ T_f, so the freezing point falls below the 5.5 °C of pure benzene.
4. So. Adding a small amount of naphthalene decreases the freezing point of benzene, which is option (b).

The idea to keep. A dissolved non-volatile solute always lowers the freezing point and raises the boiling point. The change depends only on how many solute particles there are, not on what they are.

Answer from the NTA final answer key. Solution by PrepWiser.