Steve Lagree CHML 105 Experiment 22 June 25, 2008
Purpose: The purpose of this experiment is to determine the molecular weight of a solute by freezing point depression.
Theory: Raoult's law states that there is a direct proportionality between the vapor pressure lowering of a solution and the mole fraction of the solvent. So after the freezing point depression constant is found and the amount the freezing point is depressed, it is possible to calculate the molality of the solution. Using this it is possible to find the molecular weight.
Procedure: A graduated cylinder was used to measure 10.0 ml of cyclohexane, which was then poured into a clean test tube. A thermometer and wire stirrer was inserted into the tube. The test tube was put into a 400ml beaker full of water and ice. The thermometer was monitored at 30-second intervals until the stirrer is frozen in place. When the solution freezes it looks crystallized and the thermometer refuses to move in the tube. The temperature at this point is the freezing point of cyclohexane. Then the cyclohexane was allowed to melt completely in order to remove the thermometer. 0.2g of pdichlorobenzene was added to the cyclohexane and mixed well. Then the test tube with the solution was re-inserted into the ice bath to freeze again, while the temperatures were recorded. This method was repeated with the exception that an unknown was used in place of pdichlorobenzene.
Data Part A: Trial 1 Trial 2 1. Volume of Cyclohexane 10ml 10ml 2. Weight of tare + dichlorobenzene .192g .210g 3. Weight of tare 0g 0g 4. Freezing point of cyclohexane 6.2* 6.2* 5. Freezing point of solution 1.5* 1.3*
Calculations Part A: A. Weight of Cyclohexane 7.781 7.781 10.00ml x .7781g/ml
B. Weight of dichlorobenzene .192g .210g .192 – 0 = .192 .210 – 0 = .210
C. Moles of dichlorobenzene .001306 .001429 .192/ 147 = .001306 .210 / 147 = .001429
D. Molality of solution .1678 .183597 .001396/.007781 = .1678 .001429/.007781 = .183597
E. Temperature change 4.7* 4.9* 6.2 – 1.5 = 4.7 6.2 – 1.3 = 4.9 F. Freezing point depression constant 28.01 26.69 4.7=.1678 = 28.01 4.9/.1836 = 26.69
G. Average value for freezing point depression constant 27.35 (28.01+26.69)/2 = 27.35
Data Part B Unknown Number 8 1. Volume of Cyclohexane 10ml 2. Weight of tare + dichlorobenzene .212g 3. Weight of tare 0g 4. Freezing point of cyclohexane 6.2* 5. Freezing point of solution 1.6*
Calculations Part B A. Temperature Change 4.6* 6.2-1.6=4.6
B. Molality of solution .1682 4.6/27.35 = .1682
C. Moles of unknown .001309 .1682 x .007781 = .001309
D. Weight of unknown .212g .212 – 0 = .212
E. Molar mass of unknown 161.96 .212/.001309 = 161.96
Theoretical molar mass from computer 137.1
F. Percent error 18.1% (161.96-137.1)/137.1
Results and analysis: In the experiment, the molar mass was found with 18.1% error, which is less than the 36.49% intrinsice error, most of which was due to the thermometer accuracy. This shows a relatively good outcome, and displays knowledge of how to find molar mass. Error Analysis Part A 1. Volume of hexane +- .1ml .1/10 x 100 = 1% 1% 1%
2. Weight of dichlorobenzene +- .0002g .10% .10% .0002/.1920 x 100 = .1 .0002/.210 x 100 = .1
3. Freezing point of cyclohexane +- .20*C 3.2% 3.2% .2/6.2 x 100 = 3.2
4. Freezing poinr of solution +-.20*C 13.3% 15.4% .2/1.5 x 100=13.3 .2/1.3 x 100 = 15.4
Part B 1. Volume of hexane +- .1ml .1/10 x 100 = 1% 1%
2. Weight of dichlorobenzene +- .0002g .09% .0002/.212 x 100 = .09%
3. Freezing point of cyclohexane +- .20*C 3.2% .2/6.2 x 100 = 3.2
4. Freezing poinr of solution +-.20*C 12.5% .2/1.6 x 100=12.5
Total Error (Using error from trial 2 on part A) 1 + .1 + 3.2 + 15.4 + 1 + .09 + 3.2 + 12.5 = 36.49% Experimental Error is 18.1% which is less than intrinsic error.