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The purpose of this lab was to determine the molecular mass of an unknown substance through calculating the freezing point depression when the unknown substance is added as a solute to a solvent with known properties.

Butylated hydroxytoluene (BHT),
C
15
H
24
O
(s)

Para-dichlorobenzene ( p-DCB),
C
6
H
4
Cl
2(s)

Unknown substance

Large test tube

Temperature probe

Large beaker

Water

Ring stand

Utility clamp

Copper stirrer

Hot plate
1.
Fill the beaker with water and heat it to above 80°C on the hot plate
2.
Weigh approximately 8 grams of BHT into the test tube. Record the exact mass.
3.
Using the ring stand and utility clamp to hold the test tube, place the test tube into the beaker so that all of the BHT is beneath the water. Melt all of the BHT.
4.
Remove the test tube from the hot water and begin recording the temperature as it cools. Continually stir the solution with the copper stirrer.
5.
Once the BHT has frozen re-melt it in the hot water bath.
6.
Weigh approximately 1 gram of the para-dichlorobenzene. Record the exact mass.
7.
Place the para-dichlorobenzene into the test tube with the BHT and melt the mixture. Stir to make sure that it is homogeneous.
8.
Repeat step 4.
9.
Repeat steps 1-9 replacing the para-dichlorobenzene with the unknown substance.

Pure BHT
Time Temp
1.90 72.67595
2.00 72.00471
2.10 71.16346
2.20 70.56201
2.30 70.09102
2.40 69.23934
2.50 68.76451
2.60 68.28158
2.70 68.31746
2.80 68.65575
2.90 68.99979
3.00 69.30158
3.10 69.48321
3.20 69.59431
3.30 69.68285
3.40 69.73721
3.50 69.74580
3.60 69.74294
3.70 69.73721
3.80 69.74294
3.90 69.72003
4.00 69.72003
4.10 69.73148
4.20 69.72576
4.30 69.71145
4.25
4.50
4.75
5.00
5.25
5.50
5.75
6.00
BHT + Unknown
Time Temp
1.50
1.75
2.00
2.25
72.84952
71.13672
69.34974
67.71469
2.50
2.75
3.00
3.25
3.50
3.75
4.00
66.17599
64.70408
63.21260
61.79839
61.22798
61.77016
61.90207
61.98236
61.86434
61.62694
61.30468
61.09812
60.76829
60.67877
60.50714

5.75
6.00
6.25
6.50
6.75
7.00
7.25
7.50
BHT + pDCB
Time Temp
2.85
3.00
3.25
3.50
69.35541
68.53063
66.98727
65.48862
3.75
4.00
4.25
4.50
4.75
5.00
5.25
5.50
64.11544
62.54387
61.39318
60.34755
59.74862
59.96691
60.04142
59.97593
59.74413
59.45107
59.14442
58.79151
58.41956
58.10904
57.73190
57.30677
Masses used: m
BHT
(g) m p-DCB
(g) m
Unknown
(g)
Pure BHT BHT + p-DCB BHT + Unknown
8.348
N/A
N/A
8.348
1.006
N/A
8.380
N/A
1.015

The data points in the brackets were identified as supercooling and not considered in the calculations for slope or freezing point.

When the pure BHT began freezing, the temperature remained constant.

When the BHT solutions began freezing, the temperature continued to decline but at a slower rate.
Freezing point depression constant (k f
)

T f (BHT + Unkown)
= 62.959°C
∆T f
= ∆T f (BHT)
- ∆T f (BHT + Unknown)
= 69.81°C – 62.959°C = 6.85°C K f
= 9.21°C/ m
6.85°C
9.21°C/ m
= 0.744 m
K f
mol solute
M
Unknown
= ( m)(kg solvent) = (0.744 m)(8.380
E
-3 kg) = 6.23
E
-3 mol
g Unknown
M
Unknown
=
1.015 g
=
E
= 163 g/mol
Theoretical M
(Benzoic Acid) Actual M % Error
122.12 g/mol 163 g/mol 33.5%

This lab was done to provide another way of determining the molar mass of an unknown substance. The method to provide this information was through the use of colligative properties, and more specifically, freezing point depression.
When a solute is added to a solvent, the point at which the solution freezes is lower than the freezing point of the pure solvent alone. This change in temperature is equal to the freezing point depression constant multiplied by the van’t Hoff factor of the solute and the molality of the solution. Because molality is equal to moles of solute divided by kilograms of solvent, the moles of solute can be experimentally obtained by finding the change in temperature and knowing the freezing point depression constant, van’t Hoff factor, and kilograms of solvent used. Once the amount of moles is known, a simple calculation of dividing the grams of solute used by the moles of solute determined gives the molar mass of the unknown.
In this lab, the unknown substance was known to not disassociate in water, therefore making its van’t Hoff factor equal to one. The freezing point depression constant, K f
, was determined using data obtained from two known substances, BHT and para-dichlorobenzene, or pDCB. First, the freezing point of pure BHT was found by melting all of the BHT to liquid and recording the temperature as it cooled and turned solid. As the BHT changed states, the temperature leveled out at the point where there was both solid and liquid present. The intersection of the slope of this line and the slope of the decreasing temperature from the liquid BHT gave the freezing point of the pure
BHT. This process was then repeated with BHT and the pDCB, only now when the solution began to change states the temperature continued to decrease, yet at a slower rate.
The freezing point of this solution was found the same way as the pure BHT solution, and the difference between the two was the freezing point depression. The molality was calculated by dividing the moles of pDCB (grams used/molar mass) by the mass of the
BHT in kilograms. K f
could then be found by dividing the change in temperature by the molality of the BHT + pDCB solution.
After determining the freezing point depression constant and the freezing point of the pure solvent, the unknown substance was added to the BHT as the solute and the

process was repeated. The freezing point was found at the intersection of the two slopes, thus giving a way to calculate the change in temperature. From there, the moles of solute could be found by dividing the change in temperature by K f
and multiplying by the kilograms of solvent used. Finally, the experimental molar mass was calculated by dividing the grams of the unknown substance used by the moles of unknown substance.
Our final experimental molar mass of 163 g/mol was higher than the actual molar mass of benzoic acid of 122.12 g/mol, giving an error of 33.5%. There were many possible sources of error for this higher result. First, when stirring the solution as it froze to try to maintain even cooling and prevent supercooling, some of the solution, in all three cases, stuck to both the copper stirrer and the sides of the test tube. This could have affected the cooling rates of the solutions as there was less substance freezing than was calculated. Small bits of frozen solution did fall out of the test tube at one point in the experiment, also changing the mass that was cooling. In addition, friction caused by the rubbing of the copper stirrer against the temperature probe may have caused increases in temperature that made the temperature readings higher than what they actually were.
Other possible sources of error may have included particles from the air falling into the test tube, thus increasing the amount of particles in the solution, and inaccurate temperature readings from the probe touching the side of the test tube.