Statement of Purpose - DOC 4

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Statement of Purpose - DOC 4 Powered By Docstoc
					                     Sample Lab Report

                             for CHM 152

                             Lab Partner(s)


The physical properties, including the boiling point, density, and refractive index were
measured for unknown liquid #16. The infrared (IR) spectrum of the compound was also
taken. Based on the data collected, it was determined that the compound was likely to be
The purpose of this experiment was to determine the identity of an unknown organic
liquid by measuring some of its physical properties. The properties were then compared
to those of known compound to make an identification. The properties determined
included density, boiling point, refractive index, and important absorption bands in the
infrared spectrum.

Unknown number 16 was obtained for the experiment. The unknown was an oily,
colorless liquid which had a strong fruity odor. Measurement of the properties was
conducted over a period of two lab classes.

Density was determined by weighing an empty vial along with its cap. After recording
the mass, exactly 1.0 mL of unknown was transferred to the vial using an automatic
pipetting device. The vial was then quickly capped to minimize evaporation and
weighed. The mass of 1.0 mL of liquid was calculated as the difference in the two
weighings. This procedure was repeated two more times and the average of the three
measurements was recorded as the density.

The boiling point was measured using a microscale device consisting of a capillary tube,
a microcapillary tube and a melting point apparatus. The microcapillary tube was sealed
on one end by rotating the end gently in a Bunsen burner flame until it was sealed. Care
was taken to avoid overheating the tube which would cause it to bend. The larger
capillary tube was heated gently in the flame on its sealed end. While still hot, its open
end was dipped into the unknown liquid. As the tube cooled, a small amount of liquid
was drawn into it. The liquid in the tube was then tapped to the bottom of the tube. The
microcapillary tube was then placed, open-end down, into the larger tube (see Figure 1).
The tube system was then placed in a melting point apparatus where it could be observed
and heated in a controlled fashion. As the tube was heated, bubbles were observed
emerging from the mouth of the smaller tube. As the temperature rose, the bubbles
formed in a steady stream. At this point the temperature on the apparatus was turned
down and the moment the bubbling stopped, the temperature was read. This temperature
was recorded as the boiling point. This experiment was conducted three times and the
values were averaged.

The refractive index, a measure of the bending or refraction of light when it passes from
one medium into another, was measured on an Abbe Refractometer. Two or three drops
of unknown liquid was placed on the glass prisms of the instrument before the lid was
latched. While looking into the viewer, the field of view was adjusted to bring the
boundary of the lighted area and the dark area into the center of a set of crosshairs. When
this adjustment was completed the reading button was pressed and the refractive index
and the temperature were recorded. Since the refractive index is temperature sensitive,
the value was corrected for temperature with an appropriate formula (see calculations).
The infrared spectrum was run for unknown # 16. The infrared spectrum provided a plot
of the unknown molecule's tendency to absorb infrared energy, which was based on the
types of bonds present. The spectrum was used to determine the class of the compound.

Properties and Data

Weighing                         Trial 1                   Trial 2           Trial 3

Vial plus liquid                  5.52 g                       5.61 g         5.62 g
Empty vial                        4.40 g                       4.58 g         4.55 g
Difference                        1.12 g                       1.13 g         1.17 g
Average (density)               1.14 g/mL

Boiling Point

     Experiment                  Trial 1                   Trial 2           Trial 3

Boiling Point                      184                          189            188
Average                           187 o

Refractive index

      Refractive index                      Temperature

             1.578                            26.0 oC

Infrared Spectrum:

The spectrum had a strong sharp band near 1700 cm-1. This indicated either a ketone or
an aldehyde. The spectrum is shown in Figure 1 attached to this report.

The only real calculation needed was a correction for the refractive index at 26 oC.

ND corrected = ND observed - 0.000400 (T recorded - 20.0 oC)

ND corrected = 1.578 - 0.000400 (26.0 - 20.0) = 1.576
        Comparison of unknown 16 to suspected compound salicylaldehyde.

  Compound          Boiling point         Density       Refractive index       IR band

Unknown 16             187 o C           1.14 g/mL            1.576            1700 cm-1
Salicylaldhyde         196 o C           1.17 g/mL            1.574            1700 cm-1

Analysis of Error
It was observed that the measurement of physical properties appeared to include some
obvious error. Of the properties measured, it seemed apparent that the measurement of
the boiling point was the most subjective and error-prone. Knowing the precise
temperature to reduce the heat as well as correctly detecting the temperature at which
bubbling stopped was difficult. In addition, it was discovered that if the inner tube was
not perfectly sealed, the measurement was highly affected. In contrast, the measurement
of the refractive index seemed fairly straightforward, and it appeared that human error
was minimal. As the data in the table above shows, this error analysis is correct. Of the
three physical properties measured for the suspected compound, the boiling point differed
most from the literature value for the suspected compound. The difference of 9oC for the
boiling point seemed in line with the nature of the measurement.

Based on the data observed, it was determined that unknown 16 was salicylaldehyde.
Though the data did not match perfectly, there was still a good fit. The largest
discrepancy was observed for the boiling point, where an apparent 9 degree (Celsius)
error was observed. Despite the difference in boiling point values, the other measured
values matched very closely the reported values for salicylaldehyde. In addition, the
infrared spectrum shown in Figure 1 clearly indicated the presence of an aldehyde
functional group. The overall good fit of the data suggests that a proper identification has
been made.

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