Freezing Point Depression (Lab Report) PDF

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LABORATORY REPORT

PHYSICAL CHEMISTRY - CHM 271

EXPERIMENT NUMBER & TITLE

Freezing Point Depression

PREPARED BY GROUP

AS120219

DATE OF EXPERIMENT

12 MAY 2020

DATE OF SUBMISSION

15 JUNE 2020

INSTRUCTOR

INTRODUCTION The freezing point of a solvent depends upon the concentrations of the dissolved solute and the nature of the solvent. If the dissolved solute is a non-electrolyte, then the decrease in the freezing point, ΔT, is proportional to the molality, m (moles of solute per kg of solvent) of a dilute solution according to the equation: ΔT = Kf x m Where Kf is the molar freezing-point depression constant unique for each solvent. In this experiment, you will be given cyclohexane, C6H12 as a solvent and a solid that’s your molar mass will be determine from the observed freezing-point depression.

OBJECTIVES 1. To determine the freezing point of a pure solvent and a solution of an unknown. 2. To determine the molar mass of the unknown solid using freezing point depression method.

APPARATUS      

Boiling tube Thermometer Water bath Ice Glass rod Stopwatch

CHEMICALS  

Cyclohexane, C6H12 An unknown solid

PROCEDURE Part I: Freezing Point of the Solvent 1. 30 mL of cyclohexane was pour into the boiling tube. 2. The tube was placed inside a 250 mL beaker full of ice with a thermometer inside the tube. 3. Wait until the temperature dropped to 10°c before sampling the temperature for every 30 seconds until the crystal appear and the temperature reading was constant. The temperature was recorded as the freezing point of pure cyclohexane. A graph was prepared from the data obtained. The solution from procedure Part I was kept for further experiment in Part II. Part II: Sample Preparation and Freezing Point of the Mixture (solution) 1. 0.5 g of unknown was weighted as a solute into a small dry test tube. 2. The tube with the solvent and thermometer (from procedure Part I) was placed back into a beaker of boiling water (water bath)

3. After the solvent melts, the thermometer was removed and the solute sample prepare above was added into the solution. The solution was stirred well (after the solvent and the solute melts) with thermometer. 4. Heat was removed from the solution and let it cool at room temperature for about five minutes. 5. The tube was removed into an ice-filled beaker. 6. Wait until the temperature dropped to 10°c before sampling the temperature for every 30 seconds until the crystal appear and the temperature reading was constant. The temperature was recorded as the freezing point of pure cyclohexane. A graph was prepared from the data obtained

DATA TREATMENT & DATA ANALYSIS Part I: Freezing point of solvent Time (s)

0

30

60

90

120

150

180

210

240

Temp (°C)

10.0

9.3

8.8

8.4

8.2

7.7

7.0

6.5

6.5

Time (s)

270

300

330

360

Temp (°C)

6.5

6.5

6.2

6.0

Part II: Freezing point of solution Time (s)

0

30

60

90

120

150

180

210

240

Temp (°C)

10.0

8.5

7.8

7.0

6.6

6.0

5.4

4.6

3.8

Time (s)

270

300

330

360

Temp (°C)

3.5

3.5

3.5

3.2

Freezing point of solvent: 6.5 °C Freezing point of solution: 3.5 °C

Unknown solute = para-nitrotoluene

% error = =

|

|

|

|

x 100 x 100

= 1.28%

DISCUSSION The purpose of this experiment is to determine the freezing point of a pure solvent and a solution of an unknown and molar mass of the unknown solid using freezing point depression method. This experiment used Cyclohexane, C6H12 as a solvent. The freezing point is determined when the temperature become constant. For Part I of the experiment, the freezing point of the solvent is determined by melting and freezing the solution in a beaker full of ice and wait until the temperature become constant. The temperature becomes constant at 6.5°C. We found that the freezing point of cyclohexane is 6.5°C. For part II of the experiment, we combine an unknown solvent with cyclohexane. We need to melt the solvent before combine it with cyclohexane and freezing the solution together. The average freezing temperature of the solution was 3.5°C. We find that the freezing point depression of the solution is 3°C. By using the freezing point depression method, we obtained the molar mass of the solute is 138.89 gmol-1. Molar mass has the same numerical value as molecular weight of a substance. It helps to predict the unknown solute. The unknown solute is predicted as para-nitroluene which has the closest value with molar mass value which is 137.14 amu. There is 1.28% error occurs in this experiment. There is multiple cause of the error which is the solute maybe does not melted completely before the addition of cyclohexane. Besides that, we must make sure the boiling tube was submerged completely in the beaker fill with ice cubes. To reduce the error, we can repeat the experiment twice or more to get an accurate result.

CONCLUSION In conclusion, freezing point can be determined when the temperature of liquid and solid are at equilibrium, so that their vapour pressure is equal. The vapour pressure of solution can be lower than pure solvent when a non-volatile solute is added into volatile liquid solvent. These explain why the curve of freezing point of solution is different than freezing point solvent. On another side, freezing point depression is a colligative property of matter which depends on the particles present and not the type of particles or their mass. REFERENCES   

https://www.thoughtco.com/understanding-freezing-point-depression-609182 (Anne Marie Helmenstine, July 07, 2019) https://en.wikipedia.org/wiki/Freezing-point_depression# https://courses.lumenlearning.com/cheminter/chapter/freezing-point-depression/#...