Essay On Freezing Point Experiment

Name: ______________________________________ Date _________________

Lab Partner _________________________________ Section _______________

Objectives: To determine freezing points of pure solvent and solutions using cooling curves. Calculate the freezing point depression for the solutions of unknown. Calculate Molar Mass of unknown solute.

Introduction: When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is
ΔTf = i × Kf × m,
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Place the test tube with cyclohexane into the apparatus. Insert thermometer and stirring wire in it. The thermometer should not touch the bottom of the large test tube.
4. Add ice and water into the large beaker. The level of cyclohexane should be lower than the level of ice-water mixture.
5. Stir cycloxehane gently but constantly. Use large stirring rod to stir ice-water mixture constantly. Constant stirring is a key to get good results in this experiment.
6. After the temperature dropped to about 15 oC, start recording the temperature every 15 seconds. Continue recording even after cyclohexane is visually start forming solid ( you will need 5-6 measurements after you see solid in the large test tube.)
7. After you complete this part of the procedure, move large beaker down by releasing the clamps and exposing large test tube with frozen cyclohexane. Do not open the large test tube. Allow the cyclohexane to melt. You will use the same cyclohexane in the next parts of the experiment.
8. After cyclohexane completely melted, immerse the large test tube back into the fresh ice-water mixture by elevating the large beaker and securing it with clamps on the ring
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After you complete the second trial, expose the large test tube by lowering the beaker with ice-water mixture. Allow the solid to melt completely. Do not open the large test tube with cyclohexane. You will use the same cyclohexane sample in the next part of the experiment.
Measuring the Freezing Points of the Solutions
11. Measure about 0.4g of the unknown solid solute assigned to your group. Use electronic balance and weighing boat. Do not return excess amount of unknown back to its original container. Dispose in appropriate waste container instead. Record the mass of the unknown solid to the nearest 0.001g.
12. Open the large test tube. Do not remove the thermometer and the stir wire. Transfer the solid from the weighing boat into the cyclohexane, stir well to obtain clear Solution 1. All solid must dissolve, including any on the walls of the large test tube.
13. Close the large test tube, and adjust the thermometer and the stir wire. Immerse the test tube into refreshed the ice-water mixture.
14. Cool the solution to about 15 oC and start recording the temperature in 15 sec intervals. Continue cooling until the solution is frozen and you obtain at least 5 measurements after the appearance of the

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