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NEET CHEMISTRYSolutionsMedium

Question

The molal freezing point constant for water is 1.86 °C/m. Therefore, the freezing point of 0.1 m NaCl solution in water is expected to be:

A

–1.86°C

B

–0.186°C

C

–0.372°C

D

+0.372°C

Step-by-Step Solution

To find the freezing point of an electrolyte solution like NaCl, we must include the van't Hoff factor (ii) in the depression of freezing point formula: ΔTf=i×Kf×m\Delta T_f = i \times K_f \times m.

  1. Determine the van't Hoff factor (ii): NaCl is a strong electrolyte that dissociates completely in water: NaCl→Na++Cl−NaCl \rightarrow Na^+ + Cl^-. Since it yields 2 ions per formula unit, i=2i = 2.

  2. Calculate Depression in Freezing Point (ΔTf\Delta T_f): Given Kf=1.86 ∘C/mK_f = 1.86 \, ^\circ\text{C}/m and m=0.1 mm = 0.1 \text{ m}. ΔTf=2×1.86×0.1=0.372 ∘C\Delta T_f = 2 \times 1.86 \times 0.1 = 0.372 \, ^\circ\text{C}.

  3. Calculate Freezing Point of Solution (TfT_f): Tf=Tf0−ΔTfT_f = T_f^0 - \Delta T_f Freezing point of pure water (Tf0T_f^0) = 0∘C0^\circ\text{C}.

  • Tf=0−0.372=−0.372 ∘CT_f = 0 - 0.372 = -0.372 \, ^\circ\text{C}.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Solutions. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYSolutionsfreezingconstantthereforefreezingsolution

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