840509
Determination
of Molar Mass
by Vapor
Density
TEACHER’S MANUAL AND STUDENT GUIDE
Determination of Molar Mass by Vapor Density
Teacher’s Manual
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Content Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Time Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Answers to Questions in the Student Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Resource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Related Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Student Guide
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-1
Pre-Lab Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-2
Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-3
Post-Lab Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S-4
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These laboratory materials have been prepared by Carolina Biological Supply Company, which bears sole
responsibility for their contents.
©2010 Carolina Biological Supply Company
Printed in USA
Determination of Molar Mass by Vapor Density
Overview
In this lab, students determine the molar mass of the volatile liquids ethanol and
2-propanol using the Dumas method. Using this protocol, the vapor density is
calculated by measuring the mass and volume of the vapor. Students use the vapor
density, along with temperature and atmospheric pressure data, to calculate the
molar mass of each volatile liquid using the Ideal Gas Law equation.
Objectives
Students will
• use the Dumas method to determine the molar mass of volatile liquids.
• use the Ideal Gas Law to calculate the molar mass of volatile liquids.
Content Standards
This kit is appropriate for Advanced Placement® high school students and
addresses the following AP® chemistry course outline topics:
AP® Chemistry Topic Outline
II. States of Matter
A. Gases
1. Laws of ideal gases
2. Kinetic molecular theory
V. Laboratory
• Making observations of chemical reactions and substances
• Calculating and interpreting results based on the quantitative data
obtained
Recommended Experiments
Lab 3: Determination of molar mass by vapor density
Time
Requirements
Preparation | 20 minutes
Materials
The materials in this kit are sufficient for 30 students working in 10 groups
of 3 students each. The materials are supplied for use with this kit only.
Carolina Biology Supply Company disclaims all responsibility for any other
uses of these materials.
Activity
| 45 minutes
Included in the kit:
ethanol (Unknown A)
2-propanol (Unknown B)
aluminum foil
pins
Teacher’s Manual and
reproducible Student Guide
Te a c h e r ’s M a n u a l
3
Determination
of
Molar
Mass
by
Vapor
Density
Needed, but not supplied:
analytical balance accurate to
0.01 grams
10 250-mL Erlenmeyer flasks
10 1000-mL beakers
10 10-mL graduated cylinders
10 100-mL graduated cylinders
10 utility clamps
10 thermometers
Safety
10 ring stands
10 hot plates
distilled water
paper towels
labeling tape
markers
10 pairs of heat resistant gloves (or
equivalent hand protection)
Ensure that students understand and adhere to safe laboratory practices when
performing any activity in the classroom or lab. Demonstrate the protocol for
correctly using the instruments and materials necessary to complete the
activities, and emphasize the importance of proper usage. Use personal protective
equipment such as safety glasses or goggles, heat-resistant gloves, and aprons
when appropriate. Model proper laboratory safety practices for your students and
require them to adhere to all laboratory safety rules. Clean all laboratory
equipment after each use. Appropriate MSDS (Material Safety Data Sheets) are
included in this kit.
Ethanol and 2-propanol are flammable. Use a fume hood when working with
ethanol and 2-propanol. Exercise extreme caution when using either material in
the presence of a Bunsen burner flame or any other potential source of ignition.
Note: If hot plates are unavailable and you wish to conduct the activity using
Bunsen burners, ensure that students exercise caution when handling the ethanol
and 2-propanol. Supervise students closely and use Bunsen burners only if you
are certain of students’ ability to work with these materials and instruments
safely.
Have students wash their hands thoroughly with soap and water before leaving
the laboratory. Know and follow all school district guidelines for lab safety,
ventilation, and disposal of laboratory wastes.
Background
4 Te a c h e r ’s M a n u a l
A volatile liquid is one that easily can be converted to a gaseous state. The gas that
forms from the vaporization of a volatile liquid is called a vapor. A vapor is assumed
to act as an ideal gas. If the volatile liquid is unknown, utilizing the Ideal Gas Law
can determine the molar mass of the volatile liquid using the Dumas method. The
Dumas method is also referred to as the vapor density method. With the Dumas
method, the molar mass of the volatile liquid is determined by measuring the mass
of the vaporized liquid and the volume occupied by the vapor. When heated, the
vaporized liquid forces the air and excess vapors in the flask out through a pinhole
until the vapor pressure inside the flask equals the external atmospheric pressure.
Then, the volume occupied by the vapor is measured. The mass of volatile liquid is
determined from the mass of the condensed liquid in the flask. The temperature of
the gas will be the temperature of the boiling water bath. In this experiment, the
boiling point of each volatile liquid is below the boiling point of water. The pressure
will be the same as the room pressure.
Determination
of
Molar
Mass
by
Vapor
Density
The Ideal Gas Law expresses the relationship between the number of moles
of a particular substance in a gaseous state and the volume, temperature, and
pressure of that gas (Equation 1). Hence, the Ideal Gas Law can be used to
determine the molar mass of a volatile liquid from the density of the vapor.
The number of moles can be expressed as the mass of the substance (m)
divided by the molar mass of the substance (M) (Equation 2). Molar mass is
defined in terms of the mass in grams of a substance per mole of that
substance, g/mol.
Equation 1 (Ideal Gas Law):
PV = nRT
where R is the ideal gas constant, which is equal to 0.0821 (L • atm)/(mol • K).
Equation 2:
n=
m
M
If Equation 2 is substituted into Equation 1, it yields the following result:
Equation 3:
PV =
mRT
M
Solving for the molar mass of the substance yields the following equation:
Equation 4:
M=
mRT
dRT
=
PV
P
Equation 4 shows the relationship between density and molar mass. Density
equals mass divided by volume. Hence, if the vapor density can be determined
at a given temperature and pressure, then the molar mass can be calculated.
Preparation
1. Photocopy the Student Guide so that each student will have one.
Students will record their data in tables provided in the Student Guide.
2. Review the Teacher’s Manual and the Student Guide. Familiarize
yourself with the activity instructions, the required materials, and the
pre-lab and post-lab assessments.
3. Cut out 10 squares of aluminum foil, each measuring 7 centimeters per
side.
4. Place a piece of labeling tape on each bottle of ethanol and each bottle of
2-propanol. Label both bottles of ethanol “Unknown A,” and label both
bottles of 2-propanol “Unknown B.”
5. Make sure that students have access to the bottles labeled Unknown A
and Unknown B under the fume hood, and that they have access to
balances that are accurate to 0.01 grams. Then, set up a central lab
station with the following materials.
10 7-cm aluminum foil squares
10 250-mL Erlenmeyer flasks
10 10-mL graduated cylinders
10 100-mL graduated cylinders
10 1000-mL beakers
pins
paper towels
Te a c h e r ’s M a n u a l
5
Determination
of
Molar
Mass
by
Vapor
Density
6. Set up 10 workstations, one for each group. Stock each workstation with the
following materials:
ring stand
hot plate
utility clamp
thermometer
distilled water
heat-resistant gloves
Note: If hot plates are unavailable, you might wish to allow students to use
Bunsen burners under close supervision. Using Bunsen burners will require a
support ring and wire gauze for each group. Students must exercise extreme
caution when handling the ethanol and 2-propanol around a Bunsen burner flame.
Supervise students closely and use Bunsen burners only if you are certain of students’
ability to work with these materials and instruments safely.
Procedure
1. Distribute a copy of the Student Guide to each student. Have students read
the Background and then answer the Pre-Lab Questions. Review the answers
with the class.
2. Review the laboratory activity with students, and help them become familiar
with the materials and the objectives of the activity. While doing so, review
the relevant lab safety protocols.
3. Divide the class into 10 groups of 3 students each. (If your class size differs,
alter the size of the 10 groups accordingly.) Have five groups use Unknown A
and have the other five groups use Unknown B. After students have
collected the required data for their own group, but before they complete the
Post-Lab Questions, have them partner with students from a group that used
the other unknown liquid. Have them record in the appropriate table the
data collected by that group.
4. Ensure that students use appropriate personal protective equipment,
including safety goggles, lab aprons, and gloves.
5. Have students follow the instructions provided in the Student Guide, and
provide assistance as appropriate. Instruct students to record their data in the
tables in the Student Guide.
6. Know and follow all school district guidelines for lab safety, ventilation, and
disposal of laboratory wastes.
Answers to
Questions in the
Student Guide
6 Te a c h e r ’s M a n u a l
Pre-Lab Questions
1. The Dumas method also is known as “the vapor density method.” Why do
you think this is so?
Answer: With the Dumas method, molar mass is determined by measuring
the mass and the volume of the vapor. The vapor density can be calculated
from the mass and volume of the vapor, because density equals mass divided
by volume. Hence, the molar mass can be calculated using this method if the
vapor density can be determined at a given temperature and pressure.
Determination
of
Molar
Mass
by
Vapor
Density
2. What is the molar mass of a vapor if the vapor density is 2.37 g/L at
1 atm and 25°C?
Answer:
dRT (2.37 g/L × 0.0821 L • atm/mol • K × 298 K)
M=
=
= 57.9 g/mol
P
1 atm
3. What measurements must be taken to find the molar mass of a gas using
the Dumas method?
Answer: To find the molar mass of a gas using the Dumas method, the
following measurements must be taken: the mass of a known volume of
the vaporized volatile liquid, the volume occupied by the vapor, the
temperature of the water bath, and the atmospheric pressure of the room.
Post-Lab Questions
1. Complete Data Table 1 if you are working with Unknown A. Complete
Data Table 2 if you are working with Unknown B. Calculate the mass of
the unknown volatile liquid, its vapor density, and its molar mass.
Answer: Sample data is provided.
Table 1: Unknown A (Sample Data for Ethanol)
Mass of flask (g)
113.02
Mass of flask plus condensed liquid
after heating (g)
113.49
Mass of Unknown A (g)
0.45
Water bath temperature when
liquid vaporized (°C)
100
Water bath temperature when
liquid vaporized (K)
373
Volume of flask (mL)
284
Volume of flask (L)
0.284
Barometric pressure (Hg)
760
Barometric pressure (atm)
1
Vapor density (g/L)
1.6
Molar mass (g/mol)
48.52
M=
mRT (0.45 g × 0.0821 L • atm/mol • K × 373 K)
=
= 49 g/mol
PV
(1 atm × 0.284 L)
Te a c h e r ’s M a n u a l
7
Determination
of
Molar
Mass
by
Vapor
Density
Table 2: Unknown B (Sample Data for 2-Propanol)
Mass of flask (g)
112.32
Mass of flask plus condensed liquid
after heating (g)
112.85
Mass of Unknown B (g)
0.53
Water bath temperature when
liquid vaporized (°C)
100
Water bath temperature when
liquid vaporized (K)
373
Volume of flask (mL)
289
Volume of flask (L)
0.289
Barometric pressure (Hg)
760
Barometric pressure (atm)
1
Vapor density (g/L)
1.8
Molar mass (g/mol)
56
M=
mRT (0.53 g × 0.0821 L • atm/mol • K × 373 K)
=
= 56 g/mol
PV
(1 atm × 0.289 L)
2. Your instructor will provide you with the name and the actual molar mass of
each unknown volatile liquid. Record this information in Data Table 3.
Then, calculate the percent error for each unknown volatile liquid.
Answer: Sample data is provided.
Data Table 3 (Sample Data)
Unknown Volatile
Liquid
Actual
Molar Mass
(g/mol)
Experimental
Molar Mass
(g/mol)
Percent Error
ethanol
46.08
49
6.3
2-propanol
60.11
56
6.8
percent error = |experimental value – accepted value| × 100
accepted value
8 Te a c h e r ’s M a n u a l
Determination
of
Molar
Mass
percent error for ethanol =
by
Vapor
Density
|49 – 46.08|
× 100 = 6.3%
46.08
percent error for 2-propanol =
|56 – 60.11|
× 100 = 6.8%
60.11
3. Why should you ensure that the aluminum foil forms a tight seal around
the mouth of the Erlenmeyer flask during the experiment?
Answer: This is to help ensure that water does not condense under the
aluminum foil.
4. What are some possible sources of error for this experiment?
Answer: Answers may vary. Possible answers include impurities in the
compound, the liquid was not completely vaporized, there was an error
in measuring the volume of the flask, all the liquid was not allowed to
condense, and some of the vapors escaped through the pinhole.
5. Explain why the vaporized liquid is able to force the air in the flask
(along with the some of the vaporized liquid) into the environment.
Answer: The presence of the vapor in the flask creates a pressure inside
the flask that is greater than the external atmospheric pressure. This
causes the air and excess vapor to be expelled until the pressure inside
the flask equals the atmospheric pressure.
Resource
Shakhashiri, B.Z. 1983–1992. Chemical Demonstration: A Handbook for Teachers
of Chemistry, Volumes 1–4. The University of Wisconsin Press, Madison.
Related
Products
Following is a list of related items available from Carolina Biological Supply
Company. For more information, please refer to the most recent Carolina™
Science catalog, call toll free 800-334-5551, or visit our Web site at
www.carolina.com.
RN-413906 Carolina™ AP® Chemistry Molar Mass by Vapor Density CD-ROM
RN-701016 Corning® Scholar™ Hot Plate (120 V)
RN-706305B High/Low Temperature Gloves
RN-731029 12 Standard-Grade Erlenmeyer Flasks (250-mL)
Te a c h e r ’s M a n u a l
9
Student Guide
Name ____________________________
840509
Date ____________________________
AP® Chemistry
Determination of Molar Mass by Vapor Density
Background
A volatile liquid is one that easily can be converted to a gaseous state. The gas that forms from the
vaporization of a volatile liquid is called a vapor. A vapor is assumed to act as an ideal gas. If the volatile
liquid is unknown, utilizing the Ideal Gas Law can determine the molar mass of the volatile liquid using
the Dumas method. The Dumas method is also referred to as the vapor density method. With the
Dumas method, the molar mass of the volatile liquid is determined by measuring the mass of the
vaporized liquid and the volume occupied by the vapor. When heated, the vaporized liquid forces the air
and excess vapors in the flask out through a pinhole until the vapor pressure inside the flask equals the
external atmospheric pressure. Then, the volume occupied by the vapor is measured. The mass of
volatile liquid is determined from the mass of the condensed liquid in the flask. The temperature of the
gas will be the temperature of the boiling water bath. In this experiment, the boiling point of each
volatile liquid is below the boiling point of water. The pressure will be the same as the room pressure.
The Ideal Gas Law expresses the relationship between the number of moles of a particular substance in a
gaseous state and the volume, temperature, and pressure of that gas (Equation 1). Hence, the Ideal Gas
Law can be used to determine the molar mass of a volatile liquid from the density of the vapor. The
number of moles can be expressed as the mass of the substance (m) divided by the molar mass of the
substance (M) (Equation 2). Molar mass is defined in terms of the mass in grams of a substance per mole
of that substance, g/mol.
Equation 1 (Ideal Gas Law):
PV = nRT
where R is the ideal gas constant, which is equal to 0.0821( L • atm)/(mol • K).
Equation 2:
n=
m
M
If Equation 2 is substituted into Equation 1, it yields the following result:
Equation 3:
PV =
mRT
M
Solving for the molar mass of the substance yields the following equation:
Equation 4:
M=
mRT
dRT
=
PV
P
Equation 4 shows the relationship between density and molar mass. Density equals mass divided by
volume. Hence, if the vapor density can be determined at a given temperature and pressure, then the
molar mass can be calculated.
©2010 Carolina Biological Supply Company
S-1
Pre-Lab Questions
1. The Dumas method also is known as “the vapor density method.” Why do you think this is so?
2. What is the molar mass of a vapor if the vapor density is 2.37 g/L at 1 atm and 25°C?
3. What measurements must be taken to find the molar mass of a gas using the Dumas method?
Determination of Molar Mass by Vapor Density
1. Review the Background information and the Procedure before starting the lab. Wear safety goggles,
gloves, and a lab apron at all times in the laboratory. Wash your hands thoroughly with soap and
water before leaving the lab.
2. Gather the following materials from the central lab station.
aluminum foil square
250-mL Erlenmeyer flask
10-mL graduated cylinder
100-mL graduated cylinder
©2010 Carolina Biological Supply Company
1000-mL beaker
pin
paper towels
S-2
3. You should have access to the following materials at your workstation.
thermometer
distilled water
heat-resistant gloves
ring stand
hot plate
utility clamp
4. Follow the instructions in the Procedure section. Consult your instructor if you have any questions.
Procedure
1. Wipe the external surface of the 250-mL Erlenmeyer flask to remove any debris, and then mass the
flask. Record the value in the appropriate data table (Data Table 1 if using Unknown A, and Data
Table 2 if using Unknown B).
2. Cover the mouth of the Erlenmeyer flask with the 7-cm square piece of aluminum foil. Bend the foil
to form a tight seal around the mouth of the flask. This will help ensure that water does not
condense under the aluminum foil.
3. Using a pin, make a small hole in the center of the aluminum foil.
4. From under the fume hood, obtain 5 mL of your assigned unknown volatile liquid, either A or B.
5. Carefully, remove the aluminum foil and pour the 5 mL of your assigned unknown liquid (A or B)
into the flask. Replace the foil securely on the flask.
6. Fill the 1000-mL beaker with 750 mL of water, which should be enough water to cover most of the
Erlenmeyer flask when it is submerged in the beaker (see photo). Place the 1000-mL beaker on the
hot plate. Using a ring stand and utility clamp, suspend the Erlenmeyer flask and then submerge it in
the beaker of water as shown. Make certain that the water does not touch the aluminum foil
covering the mouth of the flask.
7. Heat the flask in the hot water bath until the water
boils. Let the water boil for 5–10 minutes. Adjust the
temperature of the water so that it continues to boil
but does not splash out of the beaker. The unknown
liquid should vaporize completely. Record the
temperature of the boiling water. This may vary slightly
from the boiling point of water due to impurities in the
water and/or an atmospheric pressure greater or less
than standard atmosphere pressure.
8. Carefully remove the flask from the boiling water
bath. Remember to protect your hands from the
heated flask. Allow the flask to cool for a minute or
two on the lab table.
9. Then run cold tap water over the flask to completely
cool it to room temperature. Make sure the water
does not touch the foil over the mouth of the flask.
The vapors will condense back into a liquid state.
Dumas method apparatus.
10. Thoroughly dry the outside of the flask. Mass the flask
and unknown liquid; remove the foil immediately before measuring the mass of the flask and
unknown liquid. Record the value in the appropriate data table.
©2010 Carolina Biological Supply Company
S-3
11. Use a wash bottle to rinse all solutions from the flask. Follow your teacher’s instructions for proper
disposal of laboratory wastes.
12. Measure the volume of the water in the flask using a graduated cylinder. Fill the 100-mL graduated
cylinder to 100 mL, and add the water to the flask. Repeat until the water level is even with the
mouth of the flask. Record the volume in the appropriate data table.
13. Obtain the laboratory barometric pressure from your instructor. Record the value in the appropriate
data table.
14. Before calculating the vapor density and molar mass of your assigned unknown volatile liquid,
exchange data with a group that performed the experiment using the other unknown liquid. Record
that data in the appropriate data table.
15. Answer the Post-Lab Questions.
Post-Lab Questions
1. Complete Data Table 1 if you are working with Unknown A. Complete Data Table 2 if you are
working with Unknown B. Calculate the mass of the unknown volatile liquid, its vapor density, and
its molar mass.
Data Table 1: Unknown A
Mass of flask (g)
Mass of flask plus condensed liquid
after heating (g)
Mass of Unknown A (g)
Water bath temperature when
liquid vaporized (°C)
Water bath temperature when
liquid vaporized (K)
Volume of flask (mL)
Volume of flask (L)
Barometric pressure (Hg)
Barometric pressure (atm)
Vapor density (g/L)
Molar mass (g/mol)
©2010 Carolina Biological Supply Company
S-4
Data Table 2: Unknown B
Mass of flask (g)
Mass of flask plus condensed liquid
after heating (g)
Mass of Unknown B (g)
Water bath temperature when
liquid vaporized (°C)
Water bath temperature when
liquid vaporized (K)
Volume of flask (mL)
Volume of flask (L)
Barometric pressure (Hg)
Barometric pressure (atm)
Vapor density (g/L)
Molar mass (g/mol)
2. Your instructor will provide you with the name and the actual molar mass of each unknown volatile
liquid. Record this information in Data Table 3. Then, calculate the percent error for each unknown
volatile liquid.
Data Table 3
Unknown Volatile
Liquid
Actual
Molar Mass
(g/mol)
Experimental
Molar Mass
(g/mol)
Percent Error
3. Why should you ensure that the aluminum foil forms a tight seal around the mouth of the
Erlenmeyer flask during the experiment?
©2010 Carolina Biological Supply Company
S-5
4. What are some possible sources of error for this experiment?
5. Explain why the vaporized liquid is able to force the air in the flask (along with the some of the
vaporized liquid) into the environment.
©2010 Carolina Biological Supply Company
S-6
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