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NEET CHEMISTRYSolutionsMedium

Question

A 0.1 molal aqueous solution of a weak acid (HA) is 30% ionized. If KfK_f for water is 1.86 °C/m, the freezing point of the solution will be:

A

–0.24 °C

B

–0.18 °C

C

–0.54 °C

D

–0.36 °C

Step-by-Step Solution

To solve this, we must account for the dissociation of the weak acid using the van't Hoff factor (ii).

Step 1: Calculate the van't Hoff factor (ii). The dissociation of a weak monobasic acid HA is given by: HA⇌H++A−HA \rightleftharpoons H^+ + A^- Here, the number of particles produced per molecule (nn) is 2. The degree of ionization (α\alpha) is given as 30% or 0.3. The formula for ii in case of dissociation is i=1+(n−1)αi = 1 + (n - 1)\alpha . i=1+(2−1)(0.3)=1+0.3=1.3i = 1 + (2 - 1)(0.3) = 1 + 0.3 = 1.3

Step 2: Calculate the Depression of Freezing Point (ΔTf\Delta T_f). Using the formula ΔTf=i⋅Kf⋅m\Delta T_f = i \cdot K_f \cdot m : i=1.3i = 1.3 Kf=1.86 °C/mK_f = 1.86 \text{ °C/m}

  • m=0.1 mm = 0.1 \text{ m}

ΔTf=1.3×1.86×0.1=0.2418 °C\Delta T_f = 1.3 \times 1.86 \times 0.1 = 0.2418 \text{ °C}

Step 3: Calculate the Freezing Point of the solution (TfT_f). The freezing point of pure water (Tf0T_f^0) is 0 °C. Tf=Tf0−ΔTf=0−0.2418≈−0.24 °CT_f = T_f^0 - \Delta T_f = 0 - 0.2418 \approx -0.24 \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.

CHEMISTRYSolutionsaqueoussolutionionizedfreezingsolution

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