9. Based on the information for Solution 2 and the pure benzophenone, calculate
ID: 1006428 • Letter: 9
Question
9. Based on the information for Solution 2 and the pure benzophenone, calculate the freezing point depression constant (Kf) for benzophenone. Include the units for the Kf value from your calculations.
Kf = _______________??
Info from Solution 2:
1.Mass of added benzophenone 5.049 g
2. Total mass of benzophenone in the test tube 15.102 g
3. Calculate the molality of this cyclohexanone and benzophenone solution. 0.640 M
6. freezing point (Tf) of benzophenone and cyclohexanone solution. Tf = 40.2 °C
7. freezing point temperature (?Tf) between the pure benzophenone and the benzophenone and cyclohexanone solution. ?Tf = -6.6 °C
Info from Pure Benzophenone:
1. Mass of benzophenone 10.04 g
4. From the graph prepared in question 3, determine the freezing point (Tf) of pure benzophenone. Tf* = 46.0 °C
10.Compare the freezing point depression constants for Solution 1 and Solution 2. How do these values compare to each other. Do you expect these values to be the same? Explain the reasoning behind your answer.
11.Based on your answer to question 10, are the results you obtained reasonable? What possible sources of error may occur during this experiment?
IV. Determination of the van’t Hoff factor
CaCl2 solutions
Mass of CaCl2
moles of CaCl2
Mass of solvent
molality
Tb
?Tb
i
15.075 g
0.1358
84.925 g
97.4
3.1
10.057 g
0.0906
89.943 g
94.5
6.0
1.009 g
0.0091
98.991 g
93.7
6.8
NaCl Solutions
Mass of NaCl
moles of NaCl
Mass of solvent
molality
Tb
?Tb
i
15.04 g
0.257
84.96 g
97.6
2.9
10.02 g
0.171
89.98 g
96.1
4.4
1 g
0.0171
99 g
94.9
5.6
V. Conclusions
In the space provided below, give a brief summary of your results and what you learned from this experiment. What are the main sources of error?
Experimental Kf actual Kf value %error
__________ __________ _______
Mass of CaCl2
moles of CaCl2
Mass of solvent
molality
Tb
?Tb
i
15.075 g
0.1358
84.925 g
97.4
3.1
10.057 g
0.0906
89.943 g
94.5
6.0
1.009 g
0.0091
98.991 g
93.7
6.8
Explanation / Answer
9. Kf for benzophenone
= dTf/i.m
= -6.6/1 x 0.640
= 10.31 oC/m
10.
Van't hoff factor
CaCl2 solution
1) molality = 0.1358 mol/0.084925 kg = 1.6 m
i = 3.1/1.86 x 1.6 = 1.04
2) molality = 0.0906 mol/0.089943 kg = 1.01 m
i = 6.0/1.86 x 1.01 = 3.2
3) molality = 0.0091 mol/0.098991 kg = 0.092 m
i = 6.8/1.86 x 0.092 = 39 (not valid number)
average i = 2.12
Similarly you can do for NaCl
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