Chapter-2.docx

Chapter 2 DESIGN AND METHODOLOGY The experiment on Calculation of Molar Mass of Unknown Liquid using the Density Determination via Dumas Method was performed by the students of B.S Chemical Engineering. The experiment was made successful by careful observation and well-execution of the procedures and proper handling of the apparatuses. The apparatuses used were 3 125 mL Erlenmeyer flask, 1L beaker, graduated cylinder, bunsen burner, 150°C thermometer, wash bottle, droppers, dryer and an analytical balance. The reagents used were Benzene, Distilled water, Chloroform and an Unknown liquid. Before proceeding with the experiment, the students washed and blow dried the the Erlenmeyer flasks needed in carrying out the experiment to avoid contamination that can cause an error in the results of the experiment. On the sample preparation and apparatus set up: the dry Erlenmeyer flask, small square aluminum foil and a rubber band must be weighed and recorded on the tables. Addition of about 5 mL of benzene to the flask was made then was covered with the small square aluminum foil and secured with a rubber band. The aluminum foil was pricked to make a small hole for the release of the vapor formed in the flask. The apparatus was assembled using a clamp, iron stand, bunsen burner, wire gauze, a 1L beaker filled with water, and the erlenmeyer flask. The Erlenmeyer flask was immersed in the boiling distilled water bath using a clamp and iron stand and must be submerged at least to the neck at an angle. It was also noted that there should be sufficient water in the 1L beaker to surround the bottom of the flask, but not so much that it overflows upon immersion of the flask. After which, vaporization must be performed through heating the flask until a watery or smoky like pattern has ceased to emerge from the hole in the foil. As soon as it was completed which means no more liquid in the flask and no more visible vapor has escaped through the pinhole, the temperature of the water bath was recorded in the table and the flask was removed from the bath. The outside of the flask was then carefully, thoroughly and quickly dried. It was then cooled to room temperature in order for the vapor to be condensed into a small amount of liquid. It was then eventually weighed and recorded as the mass of the flask with benzene. The preceded procedure was repeated 2 times to assure accuracy of the experiment. The procedure was also repeated for chloroform and the unknown liquid and 3 trials were also performed. As for the volume of the Erlenmeyer flask, the flask was filled up to the mouth with tap water to know its exact volume, since the volume is needed in the computation of the density of the liquid using the equation (4.1). From the values obtained, the students were able to compute for the weight of the condensed vapor by subtracting the weight of the flask with foil and rubber band from the weight of the flask with condensed vapor with foil and rubber band. After obtaining the weight of the condensed vapor and knowing the volume go the erlenmeyer flask, the density was computed. With the given data and derivation of the ideal gas equation, the molecular weight of the liquid can be solved using the equation (4.4). The mean molecular weight was computed from the molecular weight computed from each of the trials made. The mean molecular weight was then compared with the true value found in the label of the bottle of the reagent or using the Perry’s Chemical Engineers’ Handbook through the percentage error. 2

The success of this experiment was merely based on the students’ keen observation during the vaporization of the liquid and efficiency in every trial. Another contributory factor was patiently acid washing and drying of the flask to be used. Careful observation of the procedures and accuracy in computation were also a great factor in making the experiment successful.

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