Measurement of Physical Properties
Purpose:
The objective of this laboratory exercise is to acquaint the beginning Organic Chemistry
student with several common techniques used to measure physical properties of organic
compounds, namely boiling point, melting point, and density. In addition the critical skill
of purifying solids through re-crystallization will be introduced.
This laboratory exercise is scheduled over two laboratory periods and involves 4 separate
techniques. The detailed procedure for each technique is given in the associated links
below.
Procedure (Recrystallization):
You will be re-crystallizing crude salicylic acid. Make sure to take an initial melting
point, and carefully describe its condition before you begin. Pick an appropriate solvent
and re-crystallize approximately 1.0 g of material. Determine the melting point of your
'pure' salicylic acid and compare your results to those reported in the CRC handbook
and/or Aldrich catalog.
In addition, you will need to prove that your compound is really salicylic acid by
determining a mixed melting point.
Procedure (Melting Point):
Using the Mel-Temp, you will calibrate the thermometer that you will use throughout the
rest of the semester. To calibrate your thermometer, you will need to determine the
melting points for several standard materials. You will then produce a parity plot in
which you plot your observed melting point versus the literature value. Use Mr. Plot or
Excel for this so you can generate a least squares fit for your data. After you have
produced this plot, you will use it to adjust any melting or boiling points you observe and
report the 'corrected' temperature. The standard compounds you will use are the
following: p-nitrotoluene (m.p. 51.55 °C), naphthalene (m.p. 80.24 °C), benzoic acid
(m.p. 122.33 °C), and adipic acid (m.p. 151.53 °C). Don't forget to fully describe these
compounds in your observations.
Procedure (Boiling Point):
Using the regular, milli, and micro scale techniques, determine the boiling point for
methanol and ethyl benzene. Report both your observed boiling points and your
corrected boiling points. Compare your results to those reported in the CRC handbook
and/or Aldrich catalog.
Procedure (Density):
Using both the pycnometer and the syringe techniques, determine the density and specific
gravity for ethanol and methanol. Compare your results to those reported in the CRC
handbook and/or Aldrich catalog.
Conclusions:
Your conclusion should include at a minimum the following concepts:
Re-crystallization
o Explain your choice of solvent and the advantages it had compared to the other solvents
used.
o Give evidence that the re-crystallization process did or did not increase the purity of the
sample.
o If the mass of the purified product is not the same as the starting sample, explain the
changes in mass.
Melting Point
o Summarize the trend in any differences between the measured and accepted values for the
melting points of the standard compounds.
o Summarize any sources of ‘experimental error’ when determining melting point values.
Boiling Point
o Compare the results between the three different techniques
o Summarize the advantages and disadvantages of each method.
Density
o Summarize the advantages and disadvantages of each method.
o Which procedure directly measures density as opposed to specific gravity?
(Updated 08/09/2010) by J. Neilan