CHEM 201
Sec C3A
Sp 2014
Name: Douglas Miller
Date: 09-01-14
Date of Experiment: 09-03-14
Date of Submission: 09-10-14
Experiment #1: Melting and Boiling Point & Refractive Index
Abstract: Utilizing standard laboratory techniques identify the melting point and identity of an
unknown solid and the boiling point and refractive index of an unknown liquid. The melting
point was determined via the use of a Mel-Temp with sample identification being further
determined via the mixed melting point technique. Sample #115 was identified as m-toluic acid
with a melting range of 110-112.7oC. Boiling point was determined using standard techniques
with sample #397 recording a boiling point of 88oC ± 2oC. Based off the wide variation in boiling
point, it was concluded that there were either errors in the laboratory execution or possible
contamination of the sample. Refractive index was determined via the ABBE-3L Refractometer
at 20oC with a record result on sample #397 of 1.5380.
Introduction:
Part A: Utilizing standard laboratory technique an unknown sample’s melting point is to be
determined and subsequently used in identifying the sample. Melting point (mp) is defined as
the temperature at which a solid changes into its liquid state. However, rather than being an
exact point it is actually the range over which a solid melts. This is further defined as when a
solid shows the first signs of melting to the point at which all of the solid has been liquefied.
This experiment also utilizes the fact an impure solids will melt at a lower and wider range than
pure solids, this hold true even when two solids of similar melting points are mixed together.
This property when used experimentally is commonly referred to as the “mixed melting point
technique”.
This experiment calls for the selection of an unknown sample (#115). The mp of the sample is
determined via the use of a Mel-Temp and capillary tubes. Once the range has been
determined (in this case 110-112.7oC) an appropriate sample with a similar range is selected
from the table provide in Ref. (1) page 28. A 50:50 mixture of the two solids is then tested on
the Mel-Temp. If the melting point remains consistent, then according to the mixed melting
point technique it can be assumed the combine mixture is relative pure and therefore
composed of the same two products. However, should the new combine melting point be
lower and possible wider two different compounds have be mixed together and the table in
Ref. (1) page 28 should be consulted for the selection of another compound for mixture and
comparison.
Part B: Utilizing standard laboratory technique determines the boiling point of cyclohexane,
C6H12 and an unknown sample. Boiling point is defined as the temperature at which the vapor
pressure of a liquid equals atmospheric pressure. It is worth noting that as atmospheric
pressure drops with either elevation or by artificial means boiling point will also drop
accordingly, listed boiling points are generally considered to be at sea level unless otherwise
stated.
This experiment calls of the use of a sand heating bath as outlined in Ref. (1) to independently
heat both cyclohexane and an unknown (sample #397) and determine their respective boiling
points. Liquids where heated until the reflux line was halfway up the test-tube and the boiling
point was recorded when the temperature head fairly constant over a 1-minute period. The
recorded boiling point for cyclohexane was 80.2oC while unknown sample #397 was 88oC.
Part C: Utilizing standard laboratory techniques determine the refractive index of an unknown
liquid. Refractive index is a calculation that describes how light propagates though a medium.
The equation for this process is:
n= v/c
Where v is the speed of light in a vacuum and c is the speed of light through the medium.
While the refractive index is accurate down to five significant digits, it is very susceptible to
impurities and to a lesser degree temperature, so care of operation is essential. An ABBE-3L
Refractometer at 20oC using the wavelength of sodium D line was used for this experiment,
while an ISOTEMP 2006S was used to maintain the 20oC operating parameters. Sample #397
recorded a reading of 1.5380 when tested.
Experimental Procedure:
Part A: This section followed the procedures outlined in Ford & Yau (Ref. 1).
Part B: This section followed the procedures outlined in Ford & Yau (Ref. 1).
Part C: This section followed the procedures outlined in Ford & Yau (Ref. 1).
Results:
Part A: Sample #115 was selected as the unknown for this experiment. When tested it yielded
a melting range of 110-112.7oC. M-toluic acid, C8H8O2, was selected from the table in Ref. (1)
page 28 as the most similar compound with a given melting point of 113 oC. A 50:50 mixture of
the unknown and m-toluic acid yielded a melting range of 110-112.5oC
Reagents or
Products
Sample #115
Physical Properties
Experimental Values
Melting Range = 110-112.7oC
Melting Range = 110-113oC
(Ref. 2)
Melting Point = 113oC (Ref. 1)
m-toluic acid
C8H8O2
50:50 Mixture
(sample #115 &
m-toluic acid)
Literature Values
Melting Range = 110-112.5oC
Part B: When tested cyclohexane yielded a boiling point of 80.2oC. Sample #397 was selected as
the unknown for this experiment and yield a boiling point of 88 oC. During the course of boiling
sample #397 discolored into a yellow liquid, with the temperature never actual stabilizing but
fluctuating between 86-90oC. Additional heat was applied with no noticeable changes.
Retesting was not possible as insufficient sample remained.
Reagents or
Products
Sample #397
Physical Properties
Experimental Values
Literature Values
Boiling Point = 88oC ± 2oC
Sample yellowed in color during
boiling, while tempature never
actually stabilized but fluxuated
between 86-90oC.
cyclohexane
C8H12
Melting Range = 4-7oC (Ref. 2)
Boiling Point = 80.7oC (Ref. 2)
Part C: When tested sample #397 yielded a refractive index of 1.5380.
Reagents or
Products
Sample #397
Physical Properties
Experimental Values
Literature Values
Refractive Index = 1.5380
Discussion:
Part A: When tested unknown sample #115 yielded a melting range of 110-112.7oC. Comparing
this to the list of chemicals in Ref. (1) page 28, m-toluic acid was selected as the most similar in
nature with a melting point of 113oC (Ref. 1), and a melting range of 107-113oC (Ref. 2). The
50:50 mixture of unknown sample #115 and m-toluic acid resulted in a mixed melting range of
110-112.5oC. As the melting range of the 50:50 mixture was nearly identical to that of the
unknown sample it can be inferred that the mixture is a relatively pure compound. No further
tests were conducted.
Part B: While the test results from cyclohexane at 80.2oC where well within acceptable levels
the results from the unknown sample are not as clear. The yielded result on the unknown was
88oC ± 2oC, however the sample yellowed in color and never actually stabilized its temperate.
While it’s possible this is the boiling point for this sample, it does not fall within the boiling
point ranges of any of the samples listed in Ref. (1) page 28. Possible sources of the error
include:
Operator error: while the boiling point test for cyclohexane was successful it’s possible
that the unknown sample test was not correctly set up or executed. The same heating
element, sand, thermometer and other equipment was used, leading one to conclude
that the equipment was not at fault, the most likely cause may be the proximity of the
thermometer to the liquid during testing.
Contamination: As the test tube used was not actually washed prior to use but simply
pulled from the drawer and assumed to be clean, there may have been residual material
left from a previous experiment that contaminated the sample. The only other item to
come into direct contact with the sample was the boiling chips. As the operator did not
change gloves between parts A and B, there could have been carry over contamination
that was introduced when the boiling chips were added.
Expired Reagents: It’s possible that the reagents used to preparing sample #397 have
degraded or have otherwise been compromised or contaminated.
These possible errors can be corrected by first carefully following laboratory procedures and
ensuring all testing is done in accordance with the established protocol. Two ensuring that all
equipment used during testing has been properly cleaned prior to use, and gloves are either
changed between tests or do not come into direct contact with samples. In the case of boiler
chips the chips should have been poured into a secondary container then introduced into the
test tube, not poured onto the glove then added. As to expired reagents, this is an outlier and
beyond the scope of the students to correct.
Part C: Unknown sample #397 yielded a refractive index of 1.5380. The greatest challenge
while conducting this experiment was the actual reading the refractive index scale on the ABBE3L Refractometer, the operator had difficulty actually getting the index scale in focus most likely
due to their prescription.
Conclusion:
Part A: Based of the experiment and a mixed melting point of 110-112.5oC, sample #115 is most
likely m-toluic acid.
Part B: Based of the experiment the boiling point of cyclohexane is 80.2oC while sample #397
tested at 88oC ± 2oC (please see discussion for amplifying information).
Part C: Based of the experiment the refractive index for sample #397 was 1.5380.
References:
(1) Ford, E. G.; Yau, C. L. CHEM 201 Experiments in Organic Chemistry I Laboratory 8 th
Ed; Academx: Bel Air, MD, 2014; pp 27-34.
(2) www.sigmaaldrich.com (accessed 9-1-14)
(3) http://www.chemspider.com (accessed 9-1-14)
Answers to Post Lab Questions:
1. It is important to tightly pack the sample into the capillary tube to ensure even heating
of the sample and to aid to the uniform transfer of hear from the test tube walls into the
sample. Failure to properly pack the sample could result in uneven heating and/or hot
spots which would widen the melting range, given the illusion of an impure sample.
2. The melting range of a solid is recorded in lieu of the melting point because for all
practical purposes we are not dealing with a truly pure compound and small amounts of
contamination yield a wider melting “point” or range. Additionally, no matter how well
the capillary tube is packed, heat will move from the outer crystals into the inner one,
the effect of this is the outer crystals reach their melting point first followed by the
inner.
3. Heating a sample too quickly will not allow the operator to not actually capture the first
signs of liquidation and the final fully melting compound. The narrowing of the range
noted could lead one to believe that the compound in question was relative pure when
in fact it was a mixed compound.
4. The Handbook of Chemistry & Physics lists the boiling point of cyclodecanone as 106713. The “13” in this listing notes the boiling point was determined at a pressure of 13
mmHg.