CHEM 163: APPLICATION OF COLLIGATIVE PROPERTIES FOR
COMPOUND IDENTIFICATION; MOLAR MASS OF A SOLID VIA
FREEZING POINT DEPRESSION OF A SOLVENT
INTRODUCTION
Attractive intermolecular forces control many bulk physical properties of pure substances. The types
and strengths of forces possible depend on a substance’s polarity and polarizability. These
characteristics ultimately result from the type, valence, and effective nuclear charge of the atoms that
compose the substance. Simply stated, the nature of a substance will dictate its particular bulk
physical properties. Thus a pure liquid is expected to have a constant freezing point at a fixed
pressure. However, when a pure, non-volatile solute is added to pure liquid solvent the resulting
homogenous mixture freezes at a lower temperature than that of the pure liquid. This behavior is due
to the reduced vapor pressure of the solvent in solution. Interestingly, 0.01 mole each of sucrose,
vitamin-C, and caffeine dissolved separately in the same amount of water will lower water’s vapor
pressure by the same amount. The solute’s chemical nature can not be responsible for this remarkable
outcome; rather it is the number of solute particles that is significant. What causes this phenomenon?
Simply, compared to the pure solvent, the solution has lower free energy. Recall that free energy
decreases when the products have stronger bonds relative to the reactants (H < 0), and/or when
energy distributes more uniformly in the products compared to the reactants (more microstates become
available, S > 0). For a solution, the process of freezing increases the free energy because both pure
solvent and pure solute are generated and entropy decreases. Thus a lower temperature serves to
oppose this increase in free energy as the solvent freezes. The amount that the vapor pressure (free
energy) is lowered is directly related to the difference in freezing temperature between the pure solvent
and solution (Tf, C). This temperature change or freezing point depression is directly proportional to
solute
the concentration of the solution (molality (m), mol
). A quantitative function can be determined for
kg solvent
each solvent based its freezing point depression
constant (kf,

T f  mk f
kg C
).
mol
Equation 1

In this investigation masses of an unknown solute and known solvent will be measured and combined
to form a solution. The freezing point depression of the solvent will be measured and the molality of

the solution will be calculated.
With molaltity and mass of solvent known, the moles of solute and
ultimately the molar mass of the unknown can be calculated. Colligative properties of solutions thus
offer an effective means for determining molar mass. Along with freezing point depression other
colligative properties include boiling point elevation and osmotic pressure.
mass solute  k f
MM =
Equation 2
T f  mass solvent
The freezing point depression will be determined graphically from temperature vs. time plots (Figure
1) for freezing pure solvent and solution. The pure liquid’s freezing point occurs at the plateau
temperature (based on a best-fit line to the plateau data). The solution’s freezing point will be the

temperature at the intersection
of two best-fit lines, one for the initial rapid temperature decrease, and
the other for the more gradual temperature decrease that follows. Often a solution’s cooling curve has
a much less defined plateau region which requires determination of the points that are most equally
distributed about a best-fit line.
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Solidification of a Pure Solvent Compared to Its Solution with a Nonvolatile Solute
P ure So lvent-Liquid
P ure So lvent-So lidificatio n
P ure So lvent-So lid
Temperature ( o C)
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Tf
-2
-1
So lutio n-Co o ling
So lutio n-So lidificatio n
So lutio n-So lid
Linear (P ure So lvent-Liquid)
Linear (P ure So lvent-So lidificatio n)
Linear (So lutio n-Co o ling)
Linear (So lutio n-So lidificatio n)
0
1
2
3
4
5
6
7
8
9
10
11
12
Time (min)
Figure 1
PROCEDURE
SAFETY PRECAUTIONS




Cyclohexane is flammable.
No flames.
Work in the hood!
Each pair of students will be assigned a hood and will work in the same hood all quarter.
ALL WASTE MUST BE DISPOSED IN A DESIGNATED WASTE JAR
GENERAL NOTES
 The freezing point apparatus must be clean and dry.
 Get unknown and cyclohexane from the stockroom.
 Cyclohexane will be individually dispensed to prevent contamination of the stock.
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Part A: Determination of the Freezing Point of Cyclohexane
1.
2.
3.
4.
5.
6.
7.
8.
9.
Check out the freezing point apparatus. The freezing point apparatus will include a Nessler
tube (a flat bottomed tube) and a Lab Quest with two temperature probes.
Nest a 250 mL beaker in a 400 mL beaker to use for the ice bath.
Record the mass of the empty Nessler tube to the nearest tenth of a milligram.
Measure about 12 mL of cyclohexane (d = 0.779 g/mL) with a clean, dry graduated cylinder
and transfer to the Nessler tube, re-weigh and record the mass.
Cover the Nessler tube with Parafilm while setting up the LabQuest.
Plug the temperature probes in channels (CH) 1 and 2.
Turn on the Labquest. Tap “File”, then “New”.
On the right window, make sure that “Mode” is set to: Time Based.
Tap on “Rate” and set the following:



Rate: 4
Length: 20
Tap OK
Interval: 0.25
then the tap down arrow and tap min
10. Tap on the Graph Icon, then Graph, Graph Option.







Tap Autoscale
Type “Time” on the X-Axis Column
Enter Left: 0 Right: 20
Graph 1 Y-Axis, enter:
Top: 25 Bottom: 0
In the Run 1 box, check “Temperature 1 and Temperature 2”
Check “Point Protectors and Connect Points”
Tap OK.
11. Make an ice bath with a least a 3:1 ratio of ice to water. Place the temperature probe
connected to the CH2 in the ice bath.
12. Remove the Parafilm from the Nessler tube and place the CH1 probe into the cyclohexane.
13. Place the Nessler tube into the ice bath and press the start icon,
.
14. Carefully stir the cyclohexane with the probe through the freezing point determination.
15. Monitor the temperature of the ice bath to make sure that its temperature is always lower than
that for the cyclohexane. Add more ice and remove the water with a dropper to keep the ice:
water ratio 3 to 1.
16. After the determination, insert your thumb drive. Tap on “File”, click Export. Type your file
name, tap on the thumb drive icon, and press OK.
17. Cover with Parafilm until the next trial. Melt the cyclohexane by holding the Nessler tube
with your hand.
Part B: Freezing Point of Cyclohexane with Unknown Solute


Reset the LabQuest. Tap on “File”, then “New.” This will return the settings to default.
Re-enter the settings as shown above with the exception of Graph 1 Y-Axis, which
will set as follows:
Top: 25 Bottom: -15
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1. First read steps 2-9 very carefully. You should have 4 mass measurements of the tube +
contents at various stages.
2. Weigh 0.1 grams of the unknown solid to the nearest tenth of a milligram and record the mass.
3. Remove the Parafilm from the cyclohexane and re-weigh. Record the mass.
4. Quantitatively add the unknown solid to the cyclohexane. Cover with Parafilm until ready
to begin the trial. Dissolve the solid completely. If solid adheres to the wall of the tube
carefully roll the solvent along the wall to dissolve the solid. Do not bring solvent in contact
with Parafilm® as this will dissolve the Parafilm®.
5. Mix 6 mL of methanol and 18 mL of water in a beaker and then add ice. If the ice bath
temperature needs to be lowered beyond this point, add more methanol in small amounts (1 or
2 mL). Remember to replenish the ice as it melts. Fifty milliliters of methanol will lower the
temperature to about -13 oC.
6. Place the CH1 thermometer in the cyclohexane solution. Position the Nessler tube in the ice
bath and mix until freezing point is reached. Note the temperature at which solidification is
evident. Remember to monitor the ice bath’s temperature (CH2) to assure that its temperature
is lower than the cyclohexane solution.
7. After the run is complete, cover the Nessler tube with Parafilm and thaw the solution. Be
sure to warm the solution close to room temperature before starting the next trial.
8. Export the data to your thumb drive.
9. Repeat the trial two more times adding another 0.1 gram (to the nearest tenth of a milligram)
for each trial following the instructions 2-7 above. Also note any solubility changes.
GRAPHING & CALCULATIONS
Using Excel prepare three separate graphs of temperature vs. time (scatter plot, no lines). The
smallest division for the temperature axis should be 1 C and that for the time axis should be 1
minute. Each graph must show two curves; one for the pure cyclohexane and the other for a
particular solution. Properly label axes and title the graph.
Once an initial plot has been made, activate the plot, then right click inside the plot, and select
SOURCE DATA from the CHART menu. Click on the SERIES tab. Identify sets of data as
particular series, i.e. the liquid region is series 1 and the plateau region is series 2 (Initially series 1
will be all the data). Highlight the x values input cell and use the cursor to select a column of xvalues for the liquid region and then do the same for the corresponding y-values; the liquid region
is now series 1. Click the ADD SERIES button to create series 2. Now select the plateau region
data for series 2. The selection of data for these regions will depend on both the interpretation of
the initial plotted data and observations noted during each trial.
For the pure solvent trial add a trendline (best-fit) to plateau region only (series 2). The intercept
value is the freezing point of the pure solvent. For the solution trials add a trendline for the liquid
region and one for the plateau region. The intersection of these two lines corresponds to the
freezing point of the solution (Figure 1). You may need to extend the trendline (forecasting) to the
left to get a good intersection point.
From the measured data and graphical data calculate the freezing point depression, Tf, for each
trial. Calculate the molality (m) of each trial using the freezing point depression constant for
kg C
cyclohexane, kf = 20.0
. Calculate the molar mass of the unknown solid for each trial.
mol
Determine the average molar mass with standard deviation. Identify the unknown from a list of
given compounds.
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CHEM 163 Lab Report Format
Freezing Point Depression of a Solvent
The word-processed report will consist of the following 8 sections (exact order is worth points).
1. Title: Top of page: Provide the experiment title in YOUR own words (do not use the manual’s
title), the date the experiment was performed, and your name first followed by your partner’s
name.
2. Introduction and Purpose: Using YOUR own words, interest the reader in the investigation
and explain what is being determined, generally how the investigation is conducted, and why
this determination is important in the greater sense. Avoid plagiarism by citing information
that is not yours.
3. Procedure: Use the following reference:
Loftus, C.; Cabasco-Cebrian, T.; Wick, D. “Laboratory Manual for CHEM 163” spring 2011
Edition, Department of Chemistry, Seattle Central Community College.
Include the relevant page numbers for the investigation. If you change a given procedure, you
must outline, briefly, exactly what was done differently. Websites may be referenced simply,
such as www.google.com. You must also put in when the website was last updated.
4. Results: Embed any required tables into your document. Follow this section with your graphs.
For this lab you will have 1 data summary table (11R x 5C), 7 LINEST tables, and 4,
temperature vs. time plots. The summary table labels must be accurate and the table values
must respect significant digit rules and have attached units.
Trial
Quantity
Pure Solvent
Solution 1
Solution 2
Solution 3
mass of empty tube
mass of solute
NA
mass of filled tube
mass of tube contents
mass of solvent
freezing point
Tf
molality
moles of solute
molar mass of solute
NA
NA
NA
NA
The plots must have an informative and original title (never just y vs. x), properly labeled axes,
simple but visible data points, trend line equations with errors (forecast as needed), and precise
tick marks on axes. In regard to the last matter, the temperature is accurately measured to 1°C
with the most uncertain digit being in the 10ths of °C: your plot should have the smallest
division being 1°C.
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5. Calculations: Show at least one representative calculation if the calculation is used to generate
multiple values. Don’t forget significant figures and units.
6. Discussion with Error Analysis (http://seattlecentral.edu/faculty/dwick/Error
Analysis.doc):
Report the percent relative uncertainty (for precision); in the example of the CO2 molar mass,
it is 1.2 %. Note that in this particular case the “uncertainty” is a standard deviation for two
trials. Remember, standard deviation is a measure of reproducibility and is not absolute
uncertainty (the 0.001 g in 0.153 ± 0.001 g is absolute uncertainty). In this case the
experimenter chose to represent only the standard deviation and not the absolute uncertainty.
We will primarily use standard deviation in this class.
Report the percent relative error (for accuracy) if the accurate value is given; in this case of
the CO2 molar mass it is 4.3355 % (based on the accurate value of 44.008 g/mol). Discuss
sources that affect precision and accuracy; observations made during the lab are crucial here.
Suggest one or two improvements.
7. Conclusion (follow basic grammar and spelling rules): Begin by stating the main result(s)
with known uncertainty, usually standard deviation. For example, for a lab that investigated
the molar mass determination of CO2, the following would be an appropriate conclusion: “The
average molecular mass of CO2 was 42.1 ± 0.5 g/mol based on two trials. If you have values
with signs (+ or -) make a conclusion about what the sign indicates. For example, for a lab that
investigated heat of vaporization of methanol the following would be an appropriate part of the
conclusion: “The positive sign of Hº indicates the reaction is endothermic which is sensible
given that a liquid converting to a gas requires energy input to separate the molecules”.
8. Raw Data: Attach at the back of your report a Xeroxed copy of Lab Staff signed raw data
from your notebook.
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