Experiment_5_Freezing_Point_Depression

Experiment 5: Freezing Point Depression

Introduction

• Definition: Freezing-point depression is the process where adding a solute lowers the freezing point of a solvent.

• Examples:

  • Salt in water

  • Alcohol in water

  • Impurities in powdered drugs (solid-solid mixtures)

• Key Concept: The mixture (solute + solvent) has a lower freezing point than the pure solvent.

  • Real-Life Example: Seawater stays liquid below 0 °C due to dissolved salts.

Freezing Point Depression Equation

• The freezing point depression can be calculated with the formula:

  • ΔT = (Kf)(msolution)(i)

  • Where:

    • ΔT: change in freezing point

    • Kf: molar freezing point constant for the solvent

    • msolution: molality of particles in the solution

    • i: number of ions in solution (van’t Hoff factor)

• Concentration Impact:

  • Non-electrolytes (e.g., antifreeze) have a 1:1 ratio for molality and particles.

  • Electrolytes (e.g., NaCl) require calculating molality multiplied by the number of ions.

Material Needed

• 3 grams NaCl

• 250ml beaker

• Graduate cylinder

• Glass rod

• Weighing bowl

• Dropper

• Thermometer

• Ruler

• Water

Procedure

1. Measure NaCl: Use a weighing bowl to measure 3g of NaCl, and record it.

2. Prepare Solution: Add NaCl to a 250ml beaker and mix with 50.0ml water.

3. Mix Well: Stir the solution with a glass rod.

4. Test Tubes Setup: Fill one test tube with 3cm of NaCl solution and another with 3cm of water.

5. Ice-Water Bath: Create a bath using 150ml of ice and water in another beaker.

6. Add Salt: Measure 25g of NaCl to the ice-water bath and mix until at or below -10°C.

7. Temperature Recording:

   • Place water test tube in the ice-salt bath,

   • Record temperature at the top of the water (do not stir).

8. Repeat for NaCl Solution: Record the freezing temperature of the NaCl solution.