Molar Mass Determination
by Freezing Point
Depression
A case study prepared by Beyond Benign as part of the
Green Chemistry in Higher Education program: A
workshop for EPA Region 2 Colleges and Universities
2
Molar Mass Determination by Freezing Point Depression
Table of Contents
I. Summary Page 3
II. Background Page 3
III. Additional Resources for Green Chemistry in
General Chemistry and Beyond Page 4
IV. Traditional Molar Mass Determination Reaction Page 5
V. A greener approach: Molar Mass Determination by
Freezing Point Depression Page 7
VI. Conclusions and Summary Page 9
3
Molar Mass Determination by Freezing Point Depression
This experiment involves the determination of the
freezing point of a pure solvent and a solution of
an unknown organic substance dissolved in the
solvent. The molar mass of the unknown is
calculated based on the freezing point depression
of the solution.
Organic solvents are typically used in this
experiment, such as 2-methyl-2-propanol or
cyclohexane. Unknowns are used, such as
naphthalene, p-nitrotoluene, or a similar
halogenated aromatic compound. The organic
solvents have high flammability and many of the
unknowns have high human health hazards
associated with them.
Background:
This case study is a result of an EPA Region 2
Source Reduction grant
1
titled Green Chemistry in Higher Education: A Workshop for Region
2 Colleges and Universities. The Green Chemistry in Higher Education workshop was carried
out at Siena College on July 18-21, 2013. 29 faculty members participated from 20 different
institutions in New York and New Jersey. The workshop consisted of three main focus areas:
green chemistry case studies for lecture and course work, green chemistry laboratory
exercises, and toxicology and environmental impact.
During the workshop participants were able to test a variety of greener laboratory exercises
for introductory and organic chemistry courses. One of the labs was a “Greener Approach for
Measuring Colligative Properties” for the general chemistry course.
2
Three faculty members
indicated that they would be implementing the laboratory in their general chemistry courses
in the 2013 – 2014 or 2014 – 2015 academic year: Abby O’Connor, The College of New Jersey
(NJ), Elizabeth Sprague, RPI (NY), and Matthew Fountain, SUNY Fredonia (NY). Others also
expressed initial interest and therefore more are expected to adopt the lab. The reduction
in hazardous chemicals used and cost savings for this greener experiment are outlined in the
following pages.
* The waste can be reduced to close to zero if the waste is used as starting materials for other
laboratory experiments, such as biodiesel or soap making.
1
Disclaimer: Although the information in this document has been funded wholly or in part by the
United States Environmental Protection Agency under assistance agreement X9-96296312 to
Beyond Benign, it has not gone through the Agency’s publications review process and, therefore,
may not necessarily reflect the views of the Agency and no official endorsement should be
inferred.
2
Adapted from: McCarthy, S. M., and Gordon-Wylie, S. W., “A Greener Approach for Measuring
Colligative Properties”, J. Chem. Ed., 82 (1), 2005, 116-119.
Reduction in waste and
purchasing costs:
For every semester this reaction is
implemented with 100 students, there
are comparable purchasing and waste
disposal costs and an overall reduction
in waste from 0.5 gallons to 0
gallons.* The greener version of the
Molar Mass Determination also
eliminates the use of 0.33 gallons of
2-methyl-2-propanol or cyclohexane
and 0.2 lbs of one or more unknowns
such as p-nitrotoluene, naphthalene,
and 1-4-dibromobenzene, all of which
have human and/or aquatic hazards.
4
Greener Educational Materials (GEMs) Database (University of Oregon)
• Website: http://greenchem.uoregon.edu/gems.html
• Description:
by faculty members and educators world-wide
• Most curriculum is available for download (free-of-charge) or with primary literature
information
• Google map of Green Chemistry educators
American Chemical Society’s Green Chemistry Institute
• Website: www.acs.org/greenchemistry
• Description: Green Chemistry Resources for educators and students
• Experiments and Curriculum available for download
• List of ACS books on Green Chemistry
Green Chemistry Commitment
• Website: www.greenchemistrycommitment.org
•
in higher education.
• Case studies are available, university highlights, and curriculum resources
Beyond Benign
• Website: www.beyondbenign.org
• Description: Green Chemistry Curriculum available on-line (free-of-charge)
• Regional Outreach and Community Educational Events
GCEdNet – Green Chemistry Education Network
• Website: http://cmetim.ning.com/
• Description: A place where Green Chemistry educators share resources
• Blogs, discussions and chat rooms
University of Scranton Greening Across The Chemistry Curriculum
• Website: http://www.scranton.edu/faculty/cannm/green-
chemistry/english/drefusmodules.shtml
• Description: Green Chemistry modules available for download
• Power point presentations, hand-outs available
Carnegie Mellon University Institute for Green Science
• Website: http://igs.chem.cmu.edu/
• Description: Green Chemistry modules available for download
• Power point presentations, hand-outs available
5
Traditional Experiment:
Colligative Properties laboratory exercises are
commonly performed in general chemistry in
order to introduce students to colligative
properties and to use the properties to determine
the molar mass of a substance. The experiment is
typically performed with an organic solvent that
has a melting point around room temperature,
such as 2-methyl-2-propanol (25
!C), or
cyclohexane (6.5
!C). The warming curve for the
pure solvent is typically observed, followed by
the introduction of an unknown compound. The
freezing point depression is observed and can be
measured to determine the molar mass of the
unknown compound.
Molar Mass Determination by
Freezing Point Depression
Traditional Experiment
Chemicals avoided per class of
100 students:
1.25L (0.3 gal) 2-methyl-2-
propanol or 500 mL (0.13 gal)
cyclohexane
0.22 lbs of unknowns such as
naphthalene, p-nitrotoluene
and 1,4-dibromobenzene
3
“Colligative Properties: Freezing Point Depression and Molar Mass”, Experiment 19 in Chemistry The Central
Science Laboratory Experiments, 12
th
Edition, by Nelson, J.H., Kemp, K.C., and Stoltzfus, M., Pearson Education,
2012, p. 237-250.
4
Human health and aquatic toxicity data was gathered from Globally Harmonized Safety Data Sheets, which can
be obtained from Sigma-Aldrich [http://www.sigmaaldrich.com/united-states.html].
* NFPA (National Fire Protection Association) codes can be found here:
http://en.wikipedia.org/wiki/NFPA_704#Red
Chemicals used and hazards:
The chemicals that are typically used in this experiment are listed below, along with a list of
the hazards. The amounts are estimated based on a common procedure from one of the most
widely used General Chemistry textbooks
3
, along with a procedure from Monmouth University’s
General Chemistry II Laboratory Manual.
Table 1. Chemicals used, human health and aquatic toxicity data:
Chemical:
Amoun t/per/group/
of/2/stude nts:
Flammab ility:*
Human/h ealth/ toxicity:
4
Aquatic/toxicity:
4
/
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Low$toxicity$
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2?%+$-)F0+-99C&70"F3330$6I J6
Low$toxicity$$1>/302KCB 'F0L50
'+705 FDH30$6I):0M>/30
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0
8)-$$-9 )%:0
;8<=0>,?%@0A :0
8)-B'0<, C.&@0 #
"3!>
Causes$CNS$depression,$
drowsiness,$dizziness,$1,Q0-OR&%0
&,PCOC&(F01E/302,+-)F0+-&70D"FG3/0
$6IJ6:01>/302C.'F0+-&70AHF3 330$6I):0
1E/302?%+$-)F0+-99C&70"F3330$6IJ6
SC6'0&,PCOC&(@01>/302KCB'F0L50
'+70H 4/A0$6I):0M>/30
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$6I)$
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0
0
0
0
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High$toxicity$
T=U>0V+, R*0"W@0<,BBC9)(0
O-+OC.,6%.CO0&,0'R$-.B0
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2C.'F0+-&70AH 30$6I$A: 01E/30
2?%+$-)F0+-99C&70"3F3330$6IJ6$
High$toxicity$
1>/302KCB 'F0L50'+7034L#L4N0
$6I):01>/302?-*' .C-F0HN0' +70
D#A4 H0$6I) $
*#
.C&+,&,)R%.%0
"06023433HH0)970 .I-$
Moderate$toxicity$
T=U>0V+,R*0A@0;,&0O)-BBCKC-9)%0-B0
&,0C&B0O-+OC.,6%.COC&(0&,0'R$-.B40
1E/302,+-)F0+-&70"F"/30$6IJ6:01>/30
2C.'F0+-&70LG /0$6I$A$
High$toxicity$
1>/302KCB 'F0L50'+70HL4G0
$6I):0M>/302-)6-%F0L50'+70
""0$6 I)$
DFH#?C9+,$,#
9%.X%.%0
34H060234333NN0)970 .I-$
Moderate$toxicity,$causes$
respiratory$irritation0
1E/302,+-)F0$,RB %7AFD"30$ 6IJ6$
High$toxicity$
1>/302KCB 'F0L50'+70345N0$6I)$
6
Traditional Experiment, Continued:
The purchasing and waste disposal costs
associated with this procedure are estimated in
the following table. Purchasing costs were
estimated based on prices available from Sigma-
Aldrich:
5
Total amounts of chemicals used and disposed
of per class of 100 students:
• 0.33 gal of 2-methyl-2-propanol or 0.13
gal of cyclohexane
• 0.22 lbs. naphthalene, p-nitrotoluene, or
1,4-dibromobenzene
• Estimated 0.5 gallons of liquid waste*
Molar Mass Determination by
Freezing Point Depression
Traditional Experiment
Volume of waste and purchasing
and waste disposal costs per
class of 100 students:
0.5 gallons of liquid waste
$52.98-$149.12 in purchasing
and disposal costs
5
Sigma-Aldrich [http://www.sigmaaldrich.com/united-states.html, Accessed July 18, 2014].
6
Waste disposal costs are based on the EPA Cost Calculator Tool
[http://www.epa.gov/p2/pubs/resources/measurement.html#calc, accessed December 2014].
Table 2. Purchasing and waste disposal costs:
Chemical:
Amount per
100 students:
Waste
disposal
cost
6
Purchasing
cost:
5
Purchasing
cost per
100
students:
Waste
disposal
cost per
100
students:
Total cost
(per 100
students)
2-methyl-2-
propanol
1,250 mL (0.33
gal)
$11.27/gal
$100, 1L $125.00
$3.72 $128.72
Cyclohexane
500 mL (0.13
gal)
$11.27/gal
$91.40, 1 L
$45.70 $1.47 $47.17
Naphthalene
100 g (0.22 lb)
$1.35/lb
$42.20, 1 kg
$4.20
$0.30
$4.50
p-nitrotoluene
100 g (0.22 lb)
$11.27/gal
$20.10, 100
g
$20.10 $0.30 $20.40
1,4-
dibromobenzene
20 g (0.044 lb)
$1.35/lb
$28.80, 100
g
$5.76 $0.06 $5.82
TOTAL (per 100
students):
0.13 – 0.33 gal
and 0.04 – 0.22
lb
$49.90 -
$145.10
$1.53 -
$4.02
$52.98 -
$149.12
Total purchasing and waste disposal costs per class of 100 students:
• $49.90 - $145.10 in purchasing costs
• $1.53 - $4.02 in waste disposal costs
• $52.98 - $149.12 total cost
* 0.5 gallons of liquid waste is estimated due to the solid waste (naphthalene, p-nitrotoluene,
or 1-4-dibromobenzene) being dissolved in the solvent (i.e., cyclohexane) and therefore
increasing the volume of the liquid waste.
7
A Greener Approach:
The greener version of the colligative properties laboratory exercise uses fatty acids to measure
the freezing point depression of a fatty acid as an unknown is added. The greener procedure
has slightly higher purchasing costs associated with the materials (range of $36.50 - $270.70
versus $49.90 - $145.10 for the traditional procedure), depending on which of the unknown
fatty acids is used (see tables in the following pages). However, the waste can be reduced to
essentially zero since the fatty acids can be used as starting materials for other laboratory
exercises, such as a biodiesel experiment, making soap, or making wax.
Dr. Matthew Fountain at SUNY Fredonia has implemented a greener version of the typical
colligative properties laboratory experiment that uses fatty acids instead of organic solvents
and halogenated aromatic compounds.
1
The fatty acids used in the greener version of the
laboratory exercise can then be used as starting materials for other laboratory exercises, such
as making biodiesel, soap, or wax. The freezing point is first observed for pure stearic acid. An
unknown fatty acid is then added into the pure fatty acid and the freezing point depression is
observed.
Table 3. Chemicals used, human health and aquatic toxicity data:
Chemical:
Amount per
group of 2
students:
Human health toxicity:
4
Aquatic toxicity:
4
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0
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+-99C&700Y0/F3330$6IJ6
Low$toxicity$$
)-R+CO0-O C?
"06023433HH0)97
Moderate$toxicity$
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?-$-6%
High$toxicity$
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'+70A 450$6I)
*-)$C&CO0-OC?
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High$toxicity
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BJC.0C++C&-&C,.
Low$toxicity$$
* The volume of waste can be reduced to close to zero if the fatty acids are used as starting materials
for other laboratory experiments.
Molar Mass Determination by
Freezing Point Depression
A Greener Approach
Volume of waste and
purchasing and waste disposal
costs per class of 100 students:
0-1.2 lbs. of waste*
$38.14 – $272.34 in
purchasing and disposal costs
8
Molar Mass Determination by
Freezing Point Depression
A Greener Approach
Volume of waste and
purchasing and waste disposal
costs per class of 100 students:
0-1.2 lbs. of waste*
$38.14 – $272.34 in
purchasing and disposal costs
Table 4. Purchasing and waste disposal costs:
Chemical:
Amount
per 100
students:
Waste
disposal
cost
6
Purchasing
cost:
5
Purchasing
cost per
100
students:
Waste
disposal
cost per
100
students:
Total cost
(per 100
students)
stearic acid
450 g (0.99
lb)
$1.35/lb
$66.00, 1 kg
$29.70 $1.34 $31.04
lauric acid
100 g (0.22
lb)
$1.35/lb
$68.00, 1 kg
$6.80 $0.30 $7.10
palmitic acid
100 g (0.22
lb)
$1.35/lb
$241.00, 100
g
$241.00
$0.30 $241.30
myristic acid
100 g (0.22
lb)
$1.35/lb
$64.60, 100
g
$64.60 $0.30 $64.90
TOTALS:*
1.2 lbs.
$36.50 -
$270.70
$1.64
$38.14 -
$272.34
Total purchasing and waste disposal costs per class of 100 students:
• $36.50-$270.70 in purchasing costs
• $1.64 in waste disposal costs
• $38.14 - $272.34 total cost
* The volume of waste can be reduced to close to zero if the fatty acids are used as starting materials
for other laboratory experiments.
A Greener Approach, Continued:
The purchasing and waste disposal costs
associated with this procedure are estimated in
the following table. Purchasing costs were
estimated based on prices available from
Sigma-Aldrich:
5
Total amounts of chemicals used and
disposed of per class of 100 students:
• 1 lb of stearic acid
• 0.22 lbs of lauric acid, palmitic acid or
myristic acid
• 1.22 lbs of fatty acids total
• 0-1.22 lbs of waste generated*
9
Conclusions:
The greener version for measuring colligative properties of a material shows a reduction in the
hazards associated with the materials, however there are some hazards that remain for the
fatty acids (i.e., lauric acid has moderate human toxicity and high aquatic toxicity). If the
instructor limits the unknown to the safer fatty acid, myristic acid, then the hazards will be
low for the materials. The most expensive material for the greener laboratory exercise is
palmitic acid, therefore, if that unknown is avoided, then the purchasing costs can be kept
low. If the instructor uses stearic acid and myristic acid, then the following benefits can be
realized:
• Purchasing costs of $94.30 for 100 students
• Waste disposal costs of $1.64 for 1.2 pounds of waste per 100 students (However, this
can be dropped to almost $0.00 and no waste if the waste fatty acids are used as
starting materials for other experiments).
• The elimination of the organic solvents and halogenated organic compounds used in the
traditional laboratory exercise, along with the associated waste.
Traditional Experiment Summary:
Total amounts of chemicals used and
disposed of per class of 100 students:
• 0.33 gal of 2-methyl-2-propanol or
0.13 gal of cyclohexane
• 0.22 lbs. naphthalene, p-
nitrotoluene, or 1,4-dibromobenzene
•
Estimated 0.5 gallons of liquid waste
Total purchasing and waste disposal costs
per class of 100 students:
•
$49.90 - $145.10 in purchasing costs
•
$1.53 - $4.02 in waste disposal costs
• $52.98 - $149.12 total cost
A Greener Approach Summary:
Total amounts of chemicals used and
disposed of per class of 100 students:
• 1 lb of stearic acid
• 0.22 lbs of lauric acid, palmitic acid
or myristic acid
• 1.22 lbs of fatty acids total
• 0-1.22 lbs of waste generated
Total purchasing and waste disposal costs
per class of 100 students:
•
$36.50-$270.70 in purchasing costs
• $1.64 in waste disposal costs
• $38.14 - $272.34 total cost
Molar Mass Determination by
Freezing Point Depression
Summary
Waste comparison:
Eliminates use of organic
solvents and halogenated
compounds
Cost comparison:
Variable purchasing costs
depending on materials used
Molar Mass Determination by Freezing Point Depression: A case study
prepared by Beyond Benign as part of the Green Chemistry in Higher
Education program: A workshop for EPA Region 2 Colleges and Universities
Download this and other case studies at the following link:
http://www.greenchemistrycommitment.org/resources/case-studies/