INTRODUCTION: The vapor pressure of a pure liquid at a
given temperature is a characteristic property of that liquid. However, when a
nonvolatile solute is dissolved in the liquid, the vapor pressure of the liquid
is reduced. This lowering of the vapor pressure causes a change in the melting
point, boiling point, and osmotic pressure of the liquid. The magnitude of the
change in these properties depends upon the number of solute particles
dissolved in a given amount of the solvent, but not upon the nature of the
particles (their identity). Such properties are called colligative
properties. The addition of ethylene glycol to the water in a car radiator
in order to raise its boiling point or the use of salt to lower the melting
point of ice on a sidewalk are some everyday applications of colligative
properties.
In this experiment we will
investigate the phenomenon of freezing-point depression and determine the molar
mass of an unknown solute. The relationship between the lowering of the
freezing point and the concentration of a solution is given by the following:
ΔTf = Tf(pure
solvent) - Tf(solution) =
Kf · m
(1)
where Tf is the
freezing temperature, Kf is the molal freezing-point depression
constant (a property of a given solvent), and m is the molality
of solute in the solution. Molality of a solute is defined as follows:
m =
moles of solute / kg of
solvent
(2)
One
application of freezing-point depression is in the determination of the molar
mass of an unknown solute. A weighed amount of the solute is dissolved in a
known mass of solvent. The freezing point of the solvent (the temperature at
which solid and liquid phases are in equilibrium) is determined by cooling the
solution and plotting a graph of temperature as a function of time. A
horizontal portion of the graph indicates that a pure liquid will freeze at a
constant temperature. However, a solution will freeze over a range of
temperatures and will not exhibit the constant freezing point of the pure
liquid; instead of exhibiting a horizontal portion, the plot will show a change
of slope when solid solvent begins to form. The concentration of dissolved
solute then steadily increases as the solvent freezes, causing the freezing
point to continually decrease. (The solid forming is pure solvent.) The temperature at which freezing begins is
determined by the intersection of the two extrapolated straight lines. In
practice, it is necessary to first determine the freezing point of the solvent
alone, since the solvent may not be pure. This step also makes prior
calibration of the thermometer unnecessary. With the change in freezing point
and the value of Kf, it is then possible to calculate the
approximate molar mass of the solute.
The phenomenon of supercooling
occurs when a liquid cools below its freezing point without crystallizing. A
supercooled liquid is in an unstable condition, and any disturbance such as
vibration will cause crystallization to begin, with a consequent rise in
temperature to the actual freezing point. The dip in the graph due to
supercooling should be ignored and the two straight portions extrapolated to
their intersection to find the freezing point.
PRE-LAB
EXERCISE:
1. A solution containing 15.00 g
of glucose in 100.0 g of water was found to freeze at 1.53°C. Use these data
to determine the molar mass of glucose.
2. What is the expected freezing
temperature of a solution of 2.00 g of CCl4 in 100 g of benzene?
USEFUL
INFORMATION:
Table I. Freezing temperatures and constants for some
common solvents.
|
Solvent |
Formula |
Tf, °C |
Kf, °C*kg/mol |
|
Water |
H2O |
0.0 |
1.86 |
|
Acetic acid |
CH3COOH |
16.6 |
3.90 |
|
Benzene |
C6H6 |
5.5 |
5.12 |
|
tert-Butanol |
C4H9OH |
25.5 |
9.10 |
|
Cyclohexane |
C6H12 |
6.5 |
20.00 |
|
para-Dichlorobenzene |
C6H4Cl2 |
53.1 |
7.10 |
EQUIPMENT: Large test tube with two-hole
stopper and wire stirrer, large rubber stopper drilled to fit around test tube,
250-mL wide-mouth Erlenmeyer flask, powder funnel, large beaker for water bath,
100º thermometer, 2 wire test tube clamps, spatula, ring stand, iron ring, wire
gauze, 3-finger clamp, burner, flint lighter, computer, interface, and
temperature probe.
PROCEDURE: For data collection using the computer and Logger Pro software,
after starting Logger Pro, find the Setup tab on the menu bar and open
it. Check to make sure that the stainless temperature probe on channel 1
is designated as the sensor. On the data sampling tab, set the time of
sampling to 12 minutes and the sampling rate as 12 samples per minute. On
the View tab of the menu bar, you can set the axis limits under Graph
Options. The x-axis should run from zero to 12 minutes. Suggested
values for the y-axis are from 50 to 65 degrees for solvent runs and from 40 to
60 degrees for solution runs. You can modify these after the run so that
the data fill the entire graph as much as possible.
The solvent to be used is
paradichlorobenzene, C6H4Cl2, (DCB). You may
recognize from the odor that this is the substance found in mothballs. This
solvent freezes at 53.1°C if pure. Since we can not be assured of its
purity, we will measure the freezing point twice and average the values.
Weigh to the nearest 0.001 g
about 30 grams of DCB and quantitatively (completely) transfer it to the test
tube.
Fill a large beaker almost full
with hot water from the faucet. In the hood, heat the water to about 70-75°C,
but no hotter. Immerse the test tube in the water bath until most of the DCB
has melted. Insert the stopper-stirrer assembly and stir to complete the
melting of the DCB. After the sample is completely liquefied, continue
heating and stirring for an addition five minutes.
Remove the test tube from the
water bath and dry the outside. Insert the temperature probe through the second
hole of the stopper. Click the Start button on the computer screen to
begin data collection. Begin stirring the sample slowly but continuously.
Continue to stir at a steady rate until the recording of data ends.
At this point there should be a significant amount of solid present in
the test tube, but it does not have to be completely solidified. Adjust
the graph scales such that the data fill the page as much as possible and the
print the data screen to the network printer.
Remelt the sample and repeat the
freezing point determination.
Record the code number of your
unknown solute. Weigh out about 2 grams (to the nearest 0.001 g) of your
unknown and record the mass. Melt the DCB in the test tube using the hot water
bath, then carefully add the weighed sample to the test tube without losing any
of either the solute or the solvent. Heat and stir to dissolve all of the
unknown. As before, continue heating for five minutes after the solution
is homogeneous. Remove and dry the tube. Insert the temperature
probe and record time-temperature data as above (you may wish to reset the
y-axis limits beforehand). Remelt the solution and repeat the determination.
When finished, remelt the
solution, then pour it into the labeled waste jar in the hood. Do not pour
the solution down the drain. Use a small amount of toluene (FLAMMABLE)
to rinse the test tube, stirrer, and probe; discard these rinsings in the waste
solvent can. Obtain the name of your unknown compound from the instructor
and write it in your lab notebook.
CALCULATIONS: For each of the four determinations, use the data table and graph
from Logger Pro output to determine the freezing temperatures of the pure
solvent and the solution as explained in the prelab lecture (the leveling-off
point or the break in slope). Determine the average of your two trials in each
case, then use these to calculate the change in freezing point. Calculate the
molar mass of your unknown solute. Obtain the identity of your unknown from the
instructor and look up the formula to determine the correct value of the molar
mass (reference your source). Possible sources include the Handbook of
Chemistry and Physics (organic compounds section), Merck Index, Lange's
Handbook of Chemistry, or find the MSDS for the
compound. Calculate the absolute and relative errors (see the section on measurements
and error in the introduction section of the lab manual) in your result.
REPORT: Today you will write an abstract describing
the experiment and your results in 100 words or less. Use past-tense passive voice in your
abstract. Your report should also
include the data and observations from your experiment, calculations of molar
mass and a brief discussion of possible sources of error.
QUESTIONS (to be included in report)
1. Explain the effect of each of
the following errors upon your calculated molar mass. Be specific as to whether
the result would be high or low and explain why.
a. Some
solvent is lost by evaporation.
b. Some of the unknown is lost during transfer to the test tube.
c. The thermometer consistently reads one degree too low.
d. Part of the solute does not dissolve.
2. Is it necessary that the
solvent be absolutely pure? Explain your answer clearly.
3. Many ionic compounds, e.g.,
NaCl, are strong electrolytes and dissociate completely upon dissolution
in water. Explain clearly the effect upon the freezing point when a 1.00 m
solution of NaCl in water is made (calculate the freezing point). Hint: What is
the van't Hoff i-factor for this solute?