Transcript
Molecular Weight Determination Using
Freezing Point Depression
Student name:
Student number:
Lab partners:
CHE 214, Section 02
Group 3A
April 1st, 2013
TA:
Introduction and Background
Freezing point depression phenomena occur when a solution such us water which freeze at 0oC will freeze at a temperature less than that of the pure water solvent as the entropy increases by adding more solute to the solution. On the other hand, water solution will freeze at a certain temperature as the concentration of water molecule in the solution is lower than that in pure solvent and the vapor pressure of solution is equal to the vapour pressure of ice. However, the purpose of this lab was to determine and observe experimentally the molecular weight of a substance as it reach the freezing point and compare it with the actual freezing point depression by using stop watch and thermometer. Therefore, this lab divided into three parts. Firstly, the freezing point of solid benzene was observed as a 10 g of benzene placed in the inner test tube and immersed in beaker of ice, water and salt mixture. Thus, as the temperature of the thermometer was constant and the crystal begins to form the freezing point of benzene was measured. Secondly, the freezing point of the naphthalene was determined (1.1oC) as add approximately 1 g to the benzene in the inner tube and placed it in the ice water bath. Finally, the molecular weight of the unknown C by mixed it with benzene was calculated as the freezing point of the mixture was reached. The following equations below were used to compute the data;
T = T f, pure – T f, mixture
Where T f, pure is the freezing point of pure solvent in oC
T f, mixture is the freezing point of the mixture (solute/solvent) in oC
The molecular weight of the unknown C and the molal freezing point depression for the benzene were calculated using the formula shown below;
Mb= Kf Wb / T
Where Mb is the molecular weight of the solute
Wb is the molal gram solute/1000g solvent
The theoretical freezing point depression of a dissolved solute in a mixture was calculated as shown below:
T= x1 RTm2/h2m
Where x1 is the mole fraction of the solute
R is the gas constant 8.314 J/mol.k
Tm is the normal melting point of the solvent
h2m is the latent heat of melting of the solvent in Cal/mol
In conclusion to all the formulas shown above, the observation, result, graph and calculations will demonstrate each one in their own useful way.
Result and Calculations
Part 1: Determine the Freezing Point of the Benzene and Naphthalene
1190625164465Weight of Benzene: 10.0038 g
14287501447800Weight of Naphthalene: 1.0654 g
Table1. Temperature reading of each of pure Benzene and mixture of Benzene-Naphthalene in oC with respect to time (s)
Time (s)
Temperature Reading (oC) Benzene C6H6
Temperature Reading (oC) Benzene - Naphthalene (C6H6-C10H8)
0
32.3
20.9
30
30.6
19.6
60
26.6
18.4
90
23.4
17.3
120
21.0
15.8
150
19.0
15.1
180
17.5
14.2
210
15.4
13.3
240
13.7
12.5
270
12.2
11.8
300
11.0
11.1
330
9.9
10.5
360
11.3
9.8
390
9.0
9.4
420
7.8
8.9
450
6.6
8.4
480
6.0
8.0
510
4.9
7.8
540
4.2
7.2
570
3.2
6.7
600
2.5
6.3
630
1.9
5.9
660
1.3
5.5
690
0.7
5.1
720
0.3
5.3
750
-0.2
4.7
780
-0.5
4.0
810
-1.0
3.5
840
-1.2
2.9
870
-1.5
2.5
900
-1.8
2.1
930
-2.1
1.5
960
-2.3
1.0
990
-2.5
0.8
1020
-2.7
0.6
1050
-2.8
0.2
1080
-3.0
0
1110
-3.1
-0.3
1140
5.9*
1.1*
1170
5.9
1.1
* Crystal begin to Form (freezing point)
Part 2: Determine Unknown C Freezing Point
1190625164465Weight of Benzene: 10.00 g
14287501447800Weight of Unknown C: 1.06 g
Table2. Temperature Reading for the Unknown-Benzene mixture in oC with respect to time in (s)
Time (s)
Temperature (oC) of Unknown C- Benzene
0
26
60
21.5
120
19.6
180
17.8
240
15.7
300
13.8
360
12.5
420
10.7
480
9.4
540
8.2
600
7.3
660
6.6
720
6.1
780
5.6
840
4.9
870
4.4
900
4.0
930
3.6
960
3.3
990
3.1
1020
3.3
1050
3.1
1080
2.9
1110
2.9
1140
3.3
1170
3.5
1200
4.2
1230
4.3
1260
4.5
1290
3.9
1320
3.3
1350
2.9
1380
2.8
1420
2.6
1450
2.1
1480
1.9
1510
1.7
1540
1.4
1570
1.2
1600
1.1
1630
1.0
1660
0.7
1690
0.4
1720
0.3
1750
0.2
1780
1.9
1810
2.0*
1840
2.0
1870
2.0
* Crystal begin to Form (freezing point)
2. Calculations of the Freezing -Point Depression for the both Mixtures:
A. The freezing point of Naphthalene from Naphthalene-Benzene mixture:
T = T f, pure – T f, mixture T f, pure = benzene T f, mixture= Naphthalene-Benzene
= (5.9-1.1) oC
= 4.8 oC
B. The freezing point of Unknown C from Unknown C- Benzene mixture:
T = T f, pure – T f, mixture T f, mixture= Unknown C-Benzene
= (5.9-2.0) oC
= 3.9 oC
3. Calculation of the molal freezing point depression for benzene:
T for the naphthalene = 4.8 oC
Mb of naphthalene = 128.2 g/mol
Wb = g of solute C10H8/ 1000g solvent C6H6 For 10.0038 g benzene used/ 1000g = 0.01g solvent
= (1.0654 g / 10.0038 g) x1000 g solvent = 106.50 g
Mb= Kf Wb / T
Kf = (Mb x T)/ Wb
= (128.2 g/mol * 4.8 oC) / 106.50 g
= 5.78 oC/ mol
4. Calculation for the molecular weight of the unknown C:
T of the unknown C = 3.9oC
Kf for the benzene = 5.78 g .oC/ mol
Wb = (g of solute unknown C/ 10 g C6H6) x 1000g solvent
= (1.06g / 10.00 g) x 1000g solvent For 10.00 g benzene used/ 1000g solvent = 0.01g
=106 g
Mb= Kf Wb / T
= (5.78 oC/ mol x 106 g) / 3.9oC
= 157.10 g/ mol
5. Calculate T for both mixtures:
A. T for naphthalene-benzene:
n C10H8 = m / MW
= 1.0654 g / 128.2 g/mol
= 0.00831 mol
n benzene = m/ MW
= 10.0038 g/ 78.12 g/mol = 0.1281 mol
n total of mixture = n C10H8 + n C6H6
= 0.00831 mol + 0.1281 mol = 0.1364 mol
The mole fraction of the solute (naphthalene) was computed as shown below:
x1 = # of mole of solute C10H8/ total # of mole of the mixture
= 0.00831 mol/0.1364 mol = 0.0609
Tm benzene = 5.4 oC + 273.15 k = 278.55 k
h2m benzene =2375 Cal/mol 1 Cal= 4.184 J
h2m = 9937 J/mol
T= x1 RTm2/h2m
= 0.0609 *(8.314 J/mol.k)* (278.55 k) 2 / 9937 J/mol
= 3.95 K
B. T for Unknown-benzene:
n unknown C = m / MW
= 1.06 g / 157.10 g/ mol = 0.00675 mol
n benzene = m/ MW
= 10.00 g/ 78.12 g/mol = 0.1280 mol
n total of mixture = n unknown C + n C6H6
= 0.00675 mol + 0.1280 mol
= 0.1348 mol
The mole fraction of the solute (unknown C) was computed as shown below:
x1 = # of mole of solute unknown C / total # of mole of the mixture
= 0.00675 mol /0.1348 mol = 0.05
Tm = 5.4oC + 273.15 k = 278.55 k
h2m = 9937 J/mol
T= x1 RTm2/h2m
= 0.05 *(8.314 J/mol.k)* (278.55 k) 2 / 9937 J/mol
= 3.3 K
6. Calculation of Percentage error:
The values of T that was obtained experimentally and the literature were used to compute the percentage error:
Naphthalene:
Relative % error= [Literature value- experiment obtained value / Literature value] x100%
= [/ (4.0-4.8)//4.0] *100%
= 20%
Unknown C:
Relative % error = [Literature value- experiment obtained value / Literature value] x100%
= [/ (3.3-3.9)//3.3] *100%
= 18%
Plot Curves for pure substance and both mixture solutions
Figure1. The Freezing point of pure benzene, Naphthalene - Benzene and Unknown C - Benzene vs. Time in seconds
Discussion
The purpose of this experiment was to determine the molecular weight of the unknown C by measuring it mass and determine it freezing point depression. In the first part of the experiment; however, the freezing point depression of pure benzene of (10.0038 g) was first measured in order to determine the freezing point of each of naphthalene of 128.2 g/mol and unknown C (unknown molecular weight). A mass of (1.0654 g) of naphthalene solute was measured and then added to (10 g) of benzene solvent .The solution mixture was then placed in inner test tube covered by outer tube and immersed in a beaker of ice-water mixture. A small amount of salt was added to the ice-water mixture to increase the freezing point rate and thus lower the temperature of the ice-water solution. The solution naphthalene- benzene then stirred gently and thus the freezing point depression of naphthalene- benzene mixture was observed to be (1.1oC) as soon as the crystals begin to form at constant temperature reading. The freezing point of the mixture of (1.1oC) was confirmed by washing the inner tube of the solution with water to letting it to warm little bit and then refreezing by immersed back to ice-water bath and hence record the freezing point depression. Hence, naphthalene freezing point depression was calculated to be 4.8oC by subtracting the freezing point of the pure benzene to the freezing point depression of the naphthalene-benzene mixture.
The freezing point measurement for the unknown C was then determined as adding (1.06 g) of the unknown solute to (10 g) of benzene solvent. As the temperature of the solution was constant at (2.0oC) and a formed of crystal observed, the freezing point of the solution mixture unknown C- benzene was recorded. The freezing point of the unknown was then computed experimentally (3.9oC) to determine the molecular weight of the unknown C which found to be (157.10 g/ mol). However, the percentage error of the experiment was determined as calculating the theoretical freezing point of each of the naphthalene and unknown C using T= x1 RTm2/h2m… equation 5.7. For the freezing point depression of naphthalene that measured experimentally and theoretically (4.8 and 4) o C respectively the percentage error was found to 20%. However, the percentage error of the unknown was calculated in the same manner as naphthalene and was found to be 18%.
The source of error that had contributed in obtaining a low percentage error was due to mass transfer from one tube to another, the temperature reading and impurities of the solute. For example, as the solute naphthalene was added to the benzene solvent some of the mass was lost during this transfer from the tube where the mass of the solute measured to the solvent benzene test inner tube and hence cause an error in obtaining the accurate freezing point of the naphthalene. Temperature reading is one of the important factors in determining the instant occurring of freezing point depression. Due to the error in the lab performance, when the inner test tube was not covered by the outer tubes as the naphthalene was added to the benzene had contributed to instant drops in the temperature reading as soon as the naphthalene was added to the benzene and thus led to an error in determine the freezing point depression of the naphthalene. Lastly, the unknown C was impure that causes the solution of unknown-benzene to take long time to reaches it freezing point which in return causes an inaccuracy in determine the molecular weight of the unknown and hence an error in determine the freezing point depression.
Conclusions
Freezing point depression is a technique that used today in many laboratories to determine the molecular weight of the unknown by mixing it with solvent of known molecular weight such as water. However, freezing point depression occurs by lowering the vapour pressure and increasing the entropy of a solvent at a certain temperature as adding more solute to the solution. The freezing point depression of each of naphthalene and unknown substance C were obtained experimentally to be (4.8 and 3.9 oC) respectively as it mixed with benzene of known freezing point. Hence, the molecular weight of the unknown C was determined to be 157.10 g/ mol.
However, some errors had contributed in a relatively low percentage error of 20% and 18% due to error in temperature reading and lab performance such as transferring the mass as comparing the literature freezing temperature with that obtained experimentally for each of naphthalene and benzene respectively. Other than this result, the freezing point depression hypothesis was confirmed by observing the fluctuating in temperature as it decrease and then increase at the instant the solute was added to the solvent. That conclude, the freezing point depression depends on the concentration of solute added to the solvent. The more concentration of solute that expressed in molality mol of solute /1000g solvent is added the lower the freezing point.
References:
The 92st edition of the CRC Handbook of Chemistry and Physics, 2011-2012
Fred S., General Chemistry Online, Why does Salt Melt Ice?, Chemistry, 2010,
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml
Laboratory Manual for Thermodynamics II, Molecular Weight Determination using Freezing Point Depression, 2013, page 14-18.
V., Stephan (author) ‘Freezing Point Depression in Solution’, Chemistry, 2013. http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html
Wiley, H. (editor) ‘Benzene’, Table of Chemistry, 2013. http://www.stenutz.eu/chem/solv6.php?name=benzene
