Sample Lab Report for CHM 152 Name Lab Partner(s) Abstract: 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 salicylaldehyde. Introduction 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.
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The properties determined included density, boiling point, refractive index, and important absorption bands in the infrared spectrum. Experimental 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. Results Properties and Data Density |Weighing |Trial 1 |Trial 2 |Trial 3 | | | | | | |Vial plus liquid |5. 2 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. Calculations 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. 76 |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. Discussion
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.
