CAROLINA
®
84-1203
Molecular Mass by Freezing-Point
Depression
TEACHER’S GUIDE
Introduction
This activity introduces the use of probeware in the chemistry laboratory and demonstrates the benefits of probeware
over conventional methods of data acquisition. Although this activity can be done using conventional apparatus,
probeware offers superior precision and accuracy, while saving time. Because data is recorded electronically,
students can quickly tabulate or graph their data electronically and spend their time analyzing data, not managing it.
The benefits of probeware extend beyond improved data and efficiency. Probeware’s rapid data sampling rate and
extreme sensitivity allow students to study fast or subtle changes inaccessible with typical lab equipment.
Note: The following procedures are written specifically for the Palm™ m100 and an ImagiProbe™ data collection
system; however, the procedures apply to any Palm “m” series computer. With minimal modification, the procedures
may also be used with other data collection systems.
Materials
For 30 students working in pairs
Carolina Biological Supply Company product numbers are in parentheses.
15 Palm m100s (RN-41-8255)
15 ImagiProbes, “m” series (RN-41-8245A)
15 telephone plug-in adapters with DIN plugs (come with ImagiProbes)
15 temperature probes with DIN connector (RN-41-8214)
30 culture tubes, 20 ¥ 150 mm (RN-73-1513)
2 hot water baths (or hot pots)
2 round test tube racks (RN-97-4134) for hot water baths
30 safety goggles (RN-64-6704)
15 test tube holders (RN-70-2902)
15 400-mL beakers (RN-72-1210A)
15 spatulas (RN-70-2706)
30 blank labels
2 electronic balances, sensitive to 0.01 g (RN-70-2199)
lauric acid, 240 g (RN-87-1840, 100 g)
benzoic acid, 15 g (RN-84-7310, 500 g)
paper towels
blackline master for Student Guide (printed in this manual)
Preparation
Heat the water in the water baths to 90°C and put the test tube racks in the hot water baths. Place chemicals and
spatulas near balances.
Sample Problem Solution
When 2.04 g of a substance were dissolved in 7.84 g of solvent, the solution froze at 10.8°C. The pure solvent was
found to freeze at 14.3°C. The freezing-point depression constant for the solvent is 2.8°C kg/mol. Calculate the
molecular mass of the substance.
Answer:
1. Change in freezing temperature, ∆T
∆T = 14.3°C – 10.8°C = 3.5°C
2. Molality (m) of substance in the solution
m = ∆T/Kf = 3.5°C/2.8°C kg•mol = 1.2 mol/kg
©2002 Carolina Biological Supply Company
Printed in USA
Molecular Mass by Freezing-Point Depression
3. Moles of substance in solvent
moles of solute = m ¥ kg of solvent = 1.2 mol/kg ¥ 0.00784 kg = 9.4 ¥ 10–3 mol
4. Experimental molecular mass of substance
Molecular mass = grams of solute/moles of solute = 2.04 g/9.4 ¥ 10–3 mol = 220 g/mol
Sample Data
1. Mass of lauric acid (solvent): 7.98 g
2. Mass of benzoic acid (solute): 1.03 g
3. Freezing point of lauric acid: 45.6 °C (The accepted melting point of lauric acid is 44.0°C.)
4. Freezing point of benzoic/lauric acid solution: 41.2 °C
Sample Calculations
1. Change in freezing temperature, ∆T: 4.4 °C
2. Molality (m) of benzoic acid in the solution: 1.1 mol/kg
3. Moles of benzoic acid: 9.0 ¥ 10–3 mol
4. Experimental molecular mass of benzoic acid: 110 g/mol
5. Accepted molecular mass of benzoic acid, C7H6O2: 122.12 g/mol (You may wish to provide the molecular mass
of benzoic acid or ask students to calculate it as an additional exercise.)
6. Percentage of error:
|122.12 g/mol – 110 g/mol| ¥ 100 = 9.9 %
|122.12 g/mol|
3
©2002 Carolina Biological Supply Company
Student Guide
Name
84-1203
Date
Molecular Mass by Freezing-Point Depression
Objective
Use freezing-point depression to determine the molecular mass of benzoic acid.
Discussion
The freezing point of a pure substance is greater than the freezing point of the same substance in a solution.
The freezing-point depression depends on the concentration of the solute and the identity of the solvent. In
this experiment, you will determine the molecular mass of benzoic acid using the equation
∆T = m ¥ Kf
where ∆T is the freezing-point depression in degrees Celsius, m is the concentration of the solute (benzoic
acid) in the solution in units of molality (moles of solute/kg of solvent), and Kf is the molal freezing-point
depression constant of the solvent (lauric acid) in degrees Celsius kg per mole. In the experiment, you will
measure ∆T, the freezing-point depression. The value of Kf for lauric acid is 3.9°C kg/mol.
You will first calculate m, the concentration of benzoic acid in the mixture.
m = ∆T/Kf
To calculate the molecular mass, use the following relations:
m = moles of solute/kg of solvent
and moles of solute = m ¥ kg of solvent
so that molecular mass = grams of solute in solvent/moles of solute
Sample Problem
When 2.04 g of a substance was dissolved in 7.84 g of solvent, the solution froze at 10.8°C. The pure solvent
was found to freeze at 14.3°C. The freezing-point depression constant for the solvent is 2.8°C kg/mol.
Calculate the molecular mass of the substance.
Materials
Per pair of students
Palm™ m100
ImagiProbe™, “m” series
telephone plug-in adapter with DIN plug
temperature probe with DIN connector
2 culture tubes, 20 ¥ 150 mm
2 safety goggles
test tube holder
400-mL beaker
2 blank labels
paper towels
1
©2002 Carolina Biological Supply Company
Procedures
A. Freezing point of pure solvent (lauric acid)
1. Mass 8.0 g of lauric acid and record the exact mass. Place the acid in a clean culture tube labeled
#1.
2. Attach temperature probe to the DIN adapter and plug assembly into telephone jack of the
ImagiProbe. Turn on Palm and tap ImagiProbe icon. Select [New Investigation] and label it as
F.P. Depression. Select [Trial 1] and select [Preview] to monitor temperature.
3. Place the culture tube with lauric acid in the hot water bath and wait until the solid is liquefied.
4. Remove the culture tube using a test tube clamp and place in the 400-mL beaker to cool in an
upright position.
5. Insert temperature probe and stir it gently to monitor temperature.
6. When the temperature reaches 60°C, select [Collect] on the Palm screen to start recording data.
Continue stirring constantly with the probe to maintain consistent cooling.
7. Keep recording the data until the liquid solidifies and the temperature readings remain constant for
several minutes. This is the freezing point of pure lauric acid.
8. Remove the temperature probe and wipe it clean with a paper towel.
B. Freezing point of mixture (benzoic acid/lauric acid)
1. Mass 8.0 g of lauric acid and record the exact mass. Place the acid in a clean culture tube labeled
#2.
2. Mass 1.0 g of benzoic acid and record the exact mass. Place the acid in culture tube #2.
3. Select [Trial 2] under F.P. Depression investigation.
4. Place the culture tube containing lauric and benzoic acid in the hot water bath and wait until it is
liquefied.
5. Follow Steps 4–7 in Part A. In Step 7, the temperature of the solution will freeze at a lower
temperature than the pure lauric acid solvent.
Data
1. Mass of lauric acid (solvent):
2. Mass of benzoic acid (solute):
3. Freezing point of lauric acid:
g
g
°C
4. Freezing point of benzoic/lauric acid solution:
°C
Calculations
1. Change in freezing temperature, ∆T:
°C
2. Molality (m) of benzoic acid in the solution:
3. Moles of benzoic acid:
mol/kg
mol
4. Experimental molecular mass of benzoic acid:
5. Accepted molecular mass of benzoic acid, C7H6O2:
6. Percentage of error:
g/mol
g/mol
|Accepted value – Experimental value| ¥ 100 =
|Accepted value|
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2700 York Road, Burlington, North Carolina 27215
www.carolina.com
800-334-5551
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Carolina Biological Supply Company
2700 York Road, Burlington, North Carolina 27215
CB742040204