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CatalystEducation 2020

Freezing Point Depression

Objectives
• Practice efficient experimental technique while collecting temperature and time data with

the correct precision and best possible accuracy.
• Use algebra and substitution to demonstrate how molar mass can be determined using the

freezing point depression formula.
• Expand your spreadsheet graphing skills by extracting lines with two different slopes from

one data set.
• Use the intersection of lines on a graph of decreasing temperature with time to determine

freezing points of pure solvents and solutions and from those values determine the freezing
point depression to the correct precision.

• Determine molar mass from freezing point depression data.
• Analyze your technique to recognize sources of experimental error.

Introduction
When molecules come together to form a solid the process is called crystallization, freezing

or solidification. The reverse of this process is referred to as melting or fusion. A liquid containing

an impurity freezes at a different temperature than the pure liquid. This phenomenon is largely

driven by entropy, also described as the measure of disorder in a system. The more disorder in a

system, the lower its energy and the more stable the system. Just think about your room and the

energy required to mess it up compared to the energy required to organize it. A liquid solution

(containing a solvent with an impurity) is more disordered than a pure solvent. When a solution

freezes, the solvent and the impurity solidify separately, creating a relatively ordered state.

Therefore, more energy must be removed to take the solution from its highly disordered liquid

state to its ordered solid state than to do the same for a pure solvent. Hence, solutions of solvents

with impurities freeze at a lower temperature than pure solvents. Looking at the system from the

opposite direction, since the liquid solution is more disordered than the liquid pure solvent, it

requires less energy to force the solid solution into its disordered liquid state than to take the pure

solid to its liquid state, and thus the solution melts at a lower temperature than the pure solvent.

Freezing point and melting point are numerically equivalent.

The phenomenon of a lower freezing point for solutions compared to pure solvents is

referred to as freezing point depression. Freezing point depression is a colligative property and

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hence, the magnitude of the reduction in the freezing point is proportional to the quantity of impurity

present, not to the type of impurity (Eq. 1).

∆T = K! ∙ m ∙ (i)

Equation 1

where ΔT is the change in temperature between the freezing point (TFP) of the pure solvent

and the freezing point of the solvent in solution (Eq. 2).

T”#(%&'( *+,-(./) − T”#(*+,&/1+.) = ∆T

Equation 2

Kf is a constant associated with the particular solvent. The “m” represents molality, which

is defined as moles of solute divided by kilograms of solvent. Finally, van’t Hoff factor, (i), is the

effective number particles produced. (The molecule remains intact in solution; no bonds are broken

within the molecule in the process of it going into solution.) By comparison, if one mole of sodium

chloride dissolves in water, the compound dissociates into its ions and thus there are effectively

two moles of particles, one mole of sodium (Na+) ions and one mole of chloride (Cl-) ions making

the van’t Hoff factor two (i = 2 for NaCl).

Molality is moles of solute divided by kilograms of solvent and moles of solute = (mass of

solute (g) / molar mass of solute (g/mol)). Thus, you can use the freezing point depression equation

(Eq. 1) to calculate the molar mass (M) of the solute (Eq. 3) assuming that (i) = 1.

?*+,&/( =
K! ∙ m*+,&/( (g)

mass*+,-(./(kg) ∙ ∆T

Equation 2

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In this experiment, to determine the freezing point, the temperature of a system will be

measured over time as the system cools.

Figure 1. Temperature of a system as it is cooled. The temperature drops quickly as the liquid
cools and the slope of the solid line is larger. Temperature during a phase change is constant so
the slope of the dashed line should be near zero. The results for a mixture may not be quite as
distinct.

There are three steps to be performed.

Determine the freezing point of pure lauric acid (C12H24O2).

Determine the freezing point of lauric acid with benzoic acid (C7H6O2) added as a
solute.

Determine the freezing point of lauric acid with an unknown as the solute.

All experiments will be done twice so you can average the data. When you analyze the

results, you will be able to compare the molar mass of benzoic acid to its known value in the

literature. You will not be able to do the same with the unknown nor will you be able to identify

the unknown using the information obtained from the experiment.

30

40

50

60

70

80

90

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

T
em

pe
ra

tu
e
(o
C)

Time (s)

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Procedure
Accuracy and precision are critical to obtain useful data in the experiment. You should read

the thermometer to the best of your ability to the nearest 0.1ºC; this will be more difficult in the

first 30 seconds of the experiment, when the temperature is decreasing rapidly, than after the first

half minute. The time intervals of your temperature measurements must be known accurately.

Part 1: Pure Lauric Acid
1. Place ~8 grams of the solvent, lauric acid, in a weigh boat and accurately record the mass

to the

precision of the balance.

(See Videos for proper use of the electronic balances.)
2. Transfer the solid to the provided extra-large test tube, which has thicker glass walls than

you have used in the past.
3. Place a thermometer in the test tube and heat the sample in a beaker of tap water on a hot

plate until the solid has completely melted. There is no need to have a boiling water bath,
but its temperature should be near 80oC.

4. Leave the beaker with solution in the hot water bath until the temperature in the tube is 80
oC to ensure the temperature of the liquid is well above the expected freezing point.

Figure 2. Beaker set-up with wire to ensure uniform heating.

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5. While it is melting, obtain a wire loop as shown in (Figure 2). This wire will be moved

gently up and down in the liquid as it is cooling to ensure uniformity. Do NOT mix with the
thermometer because you will punch a hole through the bottom of the test tube.

6. Remove the test tube from the hot water bath and transfer it to a beaker with room
temperature tap water. Immediately record the first temperature.

7. Continue stirring with the wire, recording the temperature no less frequently than every ten
seconds (preferably every 5 seconds).

8. Continue recording temperatures until you are confident the material has solidified. The
temperature differences between each recording are less than or equal to 0.1 oC for four or
five measurements. The material may not look solid, but if the temperature remains
constant between readings, you may stop collecting data.

9. Reheat the same sample of lauric acid and repeat the experiment.

Part 2: Lauric Acid and Benzoic Acid
10. Place ~1 gram of benzoic acid in a weigh boat and accurately record the mass to the

precision of the

balance.

11. Add the benzoic acid to the test tube and place the tube back in the hot water bath.
12. Using the wire, stir the solute into the melted solvent. Leave the beaker with solution in

the hot water bath until the temperature in the tube is 80 oC to ensure the temperature of
the liquid is well above the expected freezing point and then transfer the large test tube to
the beaker of room temperature water.

13. Record the temperature as before.
14. Reheat the mixture and repeat the experiment.
15. To dispose of your material, reheat it and pour it into the proper waste container. Note:

add the solution directly to the waste container without using the funnel as the solution
may solidify in the funnel, thereby clogging the funnel. After the majority of material (>
95%) is removed from the test tube you can assist the cleaning process by using a solvent
such as acetone. Clean thoroughly and dry.

Part 3: Lauric Acid and Unknown
16. Obtain a fresh ~8g sample of lauric acid. Record the exact amount to the precision of the

balance.

17. Obtain the unknown and record the unknown number.
18. Add ~1g of unknown. Record the exact amounts of unknown and lauric acid to the

precision of the balance.

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19. Follow the procedure above to get two values for the freezing point using the unknown
so that you can average the values.

Data Analysis
To analyze the data, you will make a graph of temperature versus time for each of the six

data sets. For each data set, draw the best fit line through the data representing the cooling of the

liquid and the best fit line through the points representing the cooling of the solid. You may use

Excel to draw the trendlines (see below) or draw the lines by hand using a pen and a ruler. The

freezing point should be the point at which those two lines intersect (Figure 1). Note that the

freezing point of the solution should be lower than the freezing point of the solvent, i.e. freezing

point depression. It is always a good idea to pay attention to the data as you do the experiment.

Although you will not determine the freezing points until you create the graphs, you can estimate

the freezing points during the experiment, and you should expect the data to reveal a lower freezing

point for the solutions as compared to the pure solvent. You might check the results with your

instructor.

Making Plots and Calculating Molar Mass

1. Values for the x-axis should go to the left of data representing the y-axis in an Excel table
(Table 1), so enter time data in column A for the first trial using pure lauric acid.

Table 1 Plotting a Graph with Two Trend Lines

Time (s)
Temperature (°C)

Pure Liquid
Temperature (°C)

Pure Solid

1 0 65

2 5 60

3 10 55

4 15 50.5

5 20 48.5

6 25 47.4

7 30 46.3

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8 35 46.2

9 40 46.2

10 45 46.2

2. Determine where in your data set that the temperature remains fairly constant
(approximately 0.1 °C change in temperature) for your data for Trial 1.

3. The data before this point goes in column B as the cooling liquid data.
4. The data points where the temperature is fairly constant represents the change from liquid

lauric acid to solid lauric acid.
5. This data goes in column C, starting at the correct time that the data was collected. A

representative table of data is provided (Table 1) does not necessarily reflect the same
values or number of data points you will record.

6. Highlight these points as described in the first laboratory exercise and follow those
instructions to create a scatter plot. You will note that you have points on the graph in two
different colors representing y-values from columns B and C.

7. Right-click on one of the data points for pure liquid lauric acid to add a trendline for just
those data points.

8. Right-click on one of the data points for pure solid lauric acid to add a trendline for just
those data points.

9. To spread out the graph using the temperature interval relevant to the data you have, right
click on the y-axis labels and choose Format Axis, then you can make an appropriate
selection from the dropdown box, or you can make changes under the Layout tab using the
Axes or Gridline selection or using the plus sign in Excel 2013. See LabFlow videos if
you need more help.

10. The x-axis of the graph should start at 0 second, but the y-axis should NOT start at 0oC,
but at a temperature just below the lowest temperature recorded (Figure 1). When
formatting the axis, you should add minor tic marks or gridlines to the y-axis so that you
can maximize the precision with which you can estimate values on that axis. Make sure
you have designed the y-axis with tick marks so that you can estimate the temperature to
± 0.1 oC.

11. You should change the words in the legend to useful information by right clicking on the
legend, Select Data, highlight the series you want to name (e.g. Series 1) Click Edit, and
in the Series Name, type clarifying information, such as “pure liquid lauric acid.” Then
you can click on the next series of data (e.g. Series 2) and do the same thing, giving it a
different name.

12. When you have the graph the way you want it with appropriate titles, make a full-page
printout. You should now be able to clearly see the intersection of the trendlines for the
cooling of the liquid and the cooling of the solid.

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13. Read the temperature from the y-axis at which the two trendlines intersect to 0.1 °C for
each set of data.

14. Average the two trials for each of the three separate experiments.
15. Compute the freezing point depression for each solution (with benzoic acid and with the

unknown) using Equation 2 and the average freezing points.
16. You can then determine molar mass of each using Equation 3.

It is best to make a separate graph for each set of data collected for a total of six graphs:
Trials 1 and 2 for pure lauric acid, Trials 3 and 4 for lauric acid with benzoic acid, and Trials
5 and 6 for lauric acid with unknown.

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Freezing Point Depression Report Sheet

Name: Partner:

Instructor: Section Code: Date:

Unless otherwise noted, 4 pt. for each answer.

Clearly show how Equation 3 results from Equation 1 when (i) = 1.

Part 1: Pure Lauric Acid
Trial 1 Trial 2

Mass of Lauric Acid ___________(2 pts)

TFP (pure solvent) ___________ ___________

Average TFP (pure solvent) ___________

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Part 2: Lauric Acid and Benzoic Acid
Trial 1 Trial 2

Mass of Lauric Acid ___________

Mass of Benzoic Acid ___________(2 pts)

TFP (solution) ___________ ___________

Average TFP (solution) ___________(2 pts)

DTbenzoic acid ___________

Experimental Molar Mass of benzoic acid ___________(3 pts)

Literature value of Molar Mass of benzoic acid ___________(2 pts)

Molar mass calculation from experimental data for benzoic acid (4 pts)

Percent error calculation for molar mass of benzoic acid (4 pts)

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Part 3: Lauric Acid and Unknown
Trial 1 Trial 2

Mass of Lauric Acid ___________

Mass of Unknown ___________(2 pts)

TFP (solution) ___________ ___________
Average TFP (solution) ___________(2 pts)

DTUnknown ___________

Experimental Molar Mass of Unknown ___________(3 pts)

Molar mass calculation from experimental data for unknown (4 pts)

Error Analysis
In complete sentences, discuss the percent error in the molar mass determination for benzoic acid.

In the experiment what do you observe that might lead to error in the results? Be thorough. (6 pts.)

Graphs
Appropriately labeled graphs of all your data should be included with your report. The graphs

should be clearly notated so that the instructor can easily interpret them. Graphs must be created

independently by each individual and results on your report may come only from the graphs you

created. Do not use results from anyone else’s graphs. (20 pts.)