Chemistry 1176
Identification of an Unknown Solid
Purpose
In this experiment, you will practice a number of common
techniques used to analyze and identify unknown solid
compounds. Determining a compound’s solubility in water (a
polar solvent) and cyclohexane (a nonpolar solvent) can give
some indication whether we’re dealing with an ionic or
molecular compound, and if the compound is polar or
nonpolar. Determining the compound’s melting point is a
further indication of whether the compound is molecular (low
melting point) or ionic (high melting point), and can narrow
down the list of possible compounds dramatically. Finally,
colligative properties such as freezing point depression and
boiling point elevation allow us to accurately calculate the
molar mass of the compound. Using solubility, melting point,
and freezing point depression, we should be able to correctly
identify an unknown compound.
Introduction
When a solute is dissolved in a solvent, the freezing
temperature is lowered in proportion to the number of moles
of solute added. This property, known as freezing-point
depression, is a colligative property; that is, it depends on the
ratio of solute and solvent particles, not on the nature of the
substance itself. The equation that shows this relationship is:
t  K f  m
Figure 1
Procedure
Obtain and wear goggles.
Part I: Solubility
where t is the freezing point depression, Kf is the freezing
point depression constant for a particular solvent (20.01°C 
kg/mol for cyclohexane in this experiment), and m is the
molality of the solution (in mol solute/kg solvent).
1.
To determine the approximate solubility of a substance,
place 1–2 mL of a solvent in a 13100 mm test tube. Test
both water and cyclohexane as solvents for your
unknown.
In this experiment, you will first find the freezing temperature
of the pure solvent, cyclohexane, C6H12. You will then add a
measured mass of an unknown solute to a known mass of
cyclohexane, and determine the lowering of the freezing
temperature of the solution. In an earlier experiment, you
observed the effect on the cooling behavior at the freezing
point when a solute was added to a pure substance. By
measuring the freezing point depression, t, and the mass of
an unknown solute, you can use the formula above to find the
molecular weight of the unknown solute, in g/mole.
2.
Add a few crystals of solid, cap the test tube, and shake
vigorously. If the crystals do not dissolve, the solid is
considered insoluble. If the crystals dissolve, add more
until you get an approximate idea of whether the solid is
only slightly soluble in the solvent or very soluble.
MATERIALS
Supplies: LabQuest, Vernier temperature probe, melting point
tube (available in the laboratory with the reagents)
Reagents: Unknown solid (a compound from Table 1),
cyclohexane, ice
Part II: Melting Point
1.
To determine the melting point of the solid, place a small
amount of the unknown into a melting point tube as
instructed by your TA. You only need to fill the tube to a
height of 1–2 mm.
2.
Connect the Temperature Probe to the LabQuest and
choose New from the File menu. If you have an older
sensor that does not auto-ID, manually set up the sensor.
3.
On the Meter screen, tap Rate. Change the data-collection
rate to 1 sample/second (interval of 1 seconds/sample)
and the data-collection length to 600 seconds. Data
collection will last 10 minutes. Select OK.
4.
Use a rubber band to fasten the melting point tube to the
temperature probe such that the closed-end of the
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General Chemistry II Lab for Majors
Identification of an Unknown Solid
melting point tube is near the end of the tip of the
temperature probe. Use a utility clamp to suspend the
melting point apparatus in a warm water bath with the
rubber band above the water.
5.
Slowly heat the water bath on your hot plate until the
solid melts. Record the temperature in your data table.
6.
Repeat the procedure. This time when you approach the
unknown’s melting point, slow the rate at which you are
heating the water bath so that the temperature increases
just one degree at a time. When you reach the solid’s
melting point, all of the solid should melt at this
temperature given enough time. Record your second
measured melting point. The two measured values
should be in close agreement. If not, repeat the procedure
until your data are consistent.
Part III: Freezing Point of Cyclohexane
1.
Add about 100 mL of cool tap water to a 250-mL beaker
filled with ice. Place the beaker on the base of a ring
stand.
2.
Weigh an empty 13100 mm test tube in a 250-mL beaker
to 0.01 g and record the mass. Measure out about 5 mL of
cyclohexane in a graduated cylinder, pour it into the test
tube, and reweigh the test tube and beaker. Record the
mass. Fasten the utility clamp at the top of the test tube.
3.
Place the Temperature Probe in the test tube. Allow about
30 seconds for the temperature probe to equilibrate to the
temperature of its surroundings and give correct
temperature readings. During this time, fasten the utility
clamp to the ring stand so the test tube is positioned
above the ice bath.
4.
After the 30 seconds have elapsed, start data collection.
5.
Lower the test tube apparatus into the ice bath. Make
sure the water level outside the test tube is higher than
the cyclohexane level inside the test tube.
6.
With a very slight up and down motion of the
Temperature Probe, continuously stir the cyclohexane
during the cooling. Hold the top of the probe and not its
wire.
7.
Continue data collection until the temperature has
dropped below the freezing point and a significant
amount of the cyclohexane has crystallized. Stop data
collection and remove the test tube apparatus from the
ice bath.
8.
To determine the freezing temperature of pure
cyclohexane, you need to determine the temperature in
the portion of the graph with nearly constant
temperature. Examine the data points along this portion
of the graph. As you tap each data point, the temperature
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and time values are displayed to the right of the graph.
Record the freezing temperature of pure cyclohexane in
your data table.
9.
To determine the freezing temperature of pure
cyclohexane, you need to analyze the portion of the
graph with nearly constant temperature. To do this:
a.
Identify a flat portion of the graph.
b.
Tap and drag your stylus across the flat portion of the
graph to select the region.
c.
Choose Statistics from the Analyze menu.
d.
Record the mean (average) temperature that
represents the freezing temperature of pure
cyclohexane.
10. Store the data from the first run by tapping the File
Cabinet icon and repeat the procedure using the same
sample of cyclohexane. Use a warm water bath to melt
the solid cyclohexane and bring the solvent up to room
temperature. The two results should be in close
agreement. If they are not, repeat the procedure until
your data are consistent.
11. Remove the apparatus from the ice bath and bring the
cyclohexane back to room temperature before continuing.
Part IV: Molar Mass from Freezing Point
Depression
1.
Weigh out approximately 0.2 g of your unknown (only
0.05 g–0.10 g if using the yellow unknown) on the
analytical balance, record the sample’s mass, and then
add it to the cyclohexane in the test tube. Stir the solution
until all of the unknown solid has dissolved. Repeat Part
III Steps 3–7 to obtain the freezing point for the solution.
You may need to add some salt to the ice bath to get the
solution to crystallize.
2.
To determine the
freezing point of the
Freezing Point
solution, you need to
determine the
temperature at which
the mixture first
started to freeze.
Time
Unlike pure
cyclohexane, cooling a
mixture of a solute and cyclohexane results in a slow
gradual drop in temperature during the time period
when freezing takes place. Examine the data points to
locate the freezing point of the solution, as shown in this
figure. Record the freezing point in the Data and
Calculations table.
Chemistry 1176
Identification of an Unknown Solid
3.
A good way to compare the freezing curves of the pure
substance and the mixture is to view both sets of data on
one graph. Tap Run 2 and select All Runs. Both
temperature runs are now be displayed on the same
graph.
4.
Print a graph of temperature vs. time (with two curves
displayed). Label each curve by hand as pure cyclohexane
or unknown and cyclohexane.
Processing the Data
1.
Determine the difference in freezing temperatures, t,
between the pure cyclohexane (t1) and the mixture of
cyclohexane and your unknown (t2). Use the formula,
t  t 2  t1
2.
Calculate molality (m), in mol/kg, using the formula,
t  K fp  m
(Kfp = 20.01°C  kg/mol for cyclohexane).
3.
Calculate moles of unknown solute, using the answer in
Step 2 (in mol/kg) and the mass (in kg) of cyclohexane.
4.
Calculate the experimental molecular weight of your
unknown, in g/mol. Use the original mass of the
unknown from the Data and Calculations table, and the
moles of unknown you found in the previous step.
5.
Identify your unknown based on its melting point and
experimentally-determined molecular weight.
6.
Determine the accepted molecular weight for your
unknown from its formula. You’ll need to look this up in
your textbook or another chemistry reference text.
7.
Calculate the percent error.
.
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General Chemistry II Lab for Majors
Identification of an Unknown Solid
Table 1. Representative Solids
Possible Unknown Solids
Melting Point (°C)
Molar Mass (g/mol)
o-Nitrophenol
45
139
Benzophenone
48
182
p-Dichlorobenzene
53
147
m-Hydroxy benzyl alcohol
67
124
Biphenyl
70
154
2,4-Dimethyl anthracene
71
206
p-Nitrochlorobenzene
83
157
92.5
244
Benzil
95
210
Acenapthene
95
154
Phenanthrene
100
178
2-Napthol
122
144
Triphenylmethane
Table 2. Solvent Characteristics
Solvent
Melting Point (°C)
Kfp (°Ckg/mol)
Boiling Point (°C)
Kbp (°Ckg/mol)
Water
0.0
1.86
100.0
0.52
Benzene
5.51
5.12
80.1
2.53
Cyclohexane
6.5
20.01
81.0
2.79
Naphthalene
80.2
6.9
217.9
5.65
Phenyl ether
28.0
8.0
—
—
tert-Butyl alcohol
25.1
8.37
—
—
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Chemistry 1176
Identification of an Unknown Solid
Data and Calculations
Unknown solubility in water
Unknown solubility in cyclohexane
Melting point of unknown
°C
°C
Mass of cyclohexane
g
g
Mass of unknown
g
g
Freezing temperature of pure cyclohexane
°C
°C
Freezing point of the unknown–cyclohexane mixture
°C
°C
°C
°C
mol/kg
mol/kg
mol
mol
g/mol
g/mol
g/mol
g/mol
%
%
t
Molality, m
Moles of unknown
Molecular weight of unknown (experimental)
Identity of unknown (from Table 1, experimentallydetermined melting point, and molar mass)
Molecular weight of unknown (accepted)
Percent error
Post-Lab Questions
1.
Define the term colligative property and provide three examples of colligative properties.
2.
Calculate the freezing point of 0.5 g of biphenyl in 20.0 mL of cyclohexane.
3.
What were the main sources of error in this experiment? Name three.
4.
Between solubility, melting point, and molar mass, which is the most reliable for identifying the unknown? Support your
answer.
5.
A thermometer is miscalibrated to read 0.5 °C higher than the actual temperature over its entire scale. How would the reported
molar mass of your unknown solute be affected if this thermometer was used in determining the depressed freezing point of
cyclohexane.
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