JEE Main · 2025 · Shift-IImediumSOL-139

'x' g of NaCl is added to water in a beaker with a lid. The temperature of the system is raised from 1°C to 25°C. Which…

Solutions · Class 12 · JEE Main Previous Year Question

Question

'x' g of NaCl is added to water in a beaker with a lid. The temperature of the system is raised from 1°C to 25°C. Which out of the following plots is best suited for the change in the molarity (M) of the solution with respect to temperature? [Consider the solubility of NaCl remains unchanged over the temperature range]

Options
  1. a

    Graph (a): M decreases then increases (U-shape)

  2. b

    Graph (b): M increases to a maximum (~4°C) then decreases

  3. c

    Graph (c): M increases linearly

  4. d

    Graph (d): M decreases linearly

Correct Answerb

Graph (b): M increases to a maximum (~4°C) then decreases

Detailed Solution

Strategy:\n> Molarity (MM) depends on the volume of the solution. If the mass of solute is constant, M1/VM \propto 1/V. Relate solution volume to water's density behavior (1textC1^\circ\\text{C} to 25textC25^\circ\\text{C}).\n\nStep 1: Volume changes of water\n- 1textC\o4textC1^\circ\\text{C} \o 4^\circ\\text{C}: Density increases     \implies Volume decreases.\n- 4textC\o25textC4^\circ\\text{C} \o 25^\circ\\text{C}: Density decreases     \implies Volume increases.\n\nStep 2: Resulting Molarity\n- 1textC\o4textC1^\circ\\text{C} \o 4^\circ\\text{C}: Volume downarrow    Muparrow\\downarrow \implies M \\uparrow.\n- 4textC\o25textC4^\circ\\text{C} \o 25^\circ\\text{C}: Volume uparrow    Mdownarrow\\uparrow \implies M \\downarrow.\nMolarity peaks at 4textC4^\circ\\text{C}.\n\ntextAnswer:(2)\boxed{\\text{Answer: (2)}}

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