If the freezing point of pure water is 0°C, what will be the freezing point of a

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Question: If the freezing point of pure water is 0°C, what will be the freezing point of a 1 molal solution of NaCl? (2022)

Options:

Correct Answer: -1.86°C

Exam Year: 2022

Solution:

Freezing point depression = i * Kf * m = 2 * 1.86 * 1 = 3.72°C. Therefore, freezing point = 0 - 3.72 = -1.86°C.

If the freezing point of pure water is 0°C, what will be the freezing point of a 1 molal solution of NaCl? (2022)

If the freezing point of pure water is 0°C, what will be the freezing point of a 1 molal solution of NaCl? (2022)

Step 1: Understand that the freezing point of pure water is 0°C.
Step 2: Know that when a solute (like NaCl) is added to water, it lowers the freezing point. This is called freezing point depression.
Step 3: Identify the formula for freezing point depression: ΔTf = i * Kf * m.
Step 4: Determine the values needed for the formula: i (van 't Hoff factor for NaCl) = 2 (because NaCl dissociates into Na+ and Cl-), Kf (freezing point depression constant for water) = 1.86°C kg/mol, and m (molality of the solution) = 1 molal.
Step 5: Plug the values into the formula: ΔTf = 2 * 1.86 * 1.
Step 6: Calculate ΔTf: 2 * 1.86 * 1 = 3.72°C.
Step 7: Subtract the freezing point depression from the freezing point of pure water: 0°C - 3.72°C = -3.72°C.
Step 8: Conclude that the freezing point of a 1 molal solution of NaCl is -3.72°C.

Freezing Point Depression – The phenomenon where the freezing point of a solvent decreases when a solute is added, calculated using the formula ΔTf = i * Kf * m.
Colligative Properties – Properties that depend on the number of solute particles in a solution, not the identity of the solute.
Van 't Hoff Factor (i) – A factor that accounts for the number of particles the solute dissociates into; for NaCl, i = 2 (Na+ and Cl-).