Chem Page VII - 1 LAB MANUAL Dipole Moment 07_dipo131.docx EXPERIMENT VII

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1 Chem Page VII - 1 EXPERIMENT VII DIPOLE MOMENT OF POLAR MOLECULES IN SOLUTION (S&G 5 th ed. Expt 31, 6 th and 7 th eds. Expt. 30, 8 th ed. Expt.29) The heterodyne-beat frequency method is used to obtain a measure of the dipole moment of o- and m-dichloro-benzene. 1. Pre-Lab Preparation Work out in advance the approximate weights of dichlorobenzene required to obtain 25 ml solutions containing 1%, 2%, 3% and 4% mole fraction dichlorobenzene in benzene. 2. Safety Benzene vapours are harmful. All weighing involving benzene should be done under the fume extractor or inside a well-ventilated fume-hood. Do not discard benzene solutions to the sink; have a beaker ready in which you can dump the spent benzene samples, and keep this beaker covered. At the end of the experiment discard the benzene solutions into the proper waste solvent container. 3. Solution Preparation Good results for this experiment depend critically on accurate preparation of solutions, on good temperature control and on careful measurement of capacitances. A dedicated set of 25 ml volumetric flasks is reserved for this experiment. Ask your instructor for these flasks. Make sure that they are dry and do not use water to clean them (just rinse with acetone and air dry if needed). At the end of the experiment, return these flasks, unstoppered, to the proper drawer or tray. One piece of information needed to analyse the results is the actual density of the various solutions. Consequently to prepare the solutions, the following weighing should be performed 1 weigh the dry empty 25 ml volumetric flask (with its stopper) inject the required amount of solute weigh (stopper in) inject the solvent (Benzene) up to the 25 ml mark weigh again (with stopper in) Now you have all the information necessary to calculate the mole fraction X2 1 While weighing a liquid in a flask, the mouth of the flask and the stopper must be dry otherwise evaporation occurs resulting in unstable weight reading.

2 Page VII - 2 Chem X 2 = #moles solute #moles solute + # moles solvent and d, the density of each sample solution (1) mass of solution d = (2) volume of solution Remember to prepare a blank solution (pure benzene) in the same way as the others and perform the same series of measurements on this blank. The corresponding data will give you an extra point on all your plots (zero mole fraction dichlorobenzene). 4. Capacitance Measurements First hang all the flasks containing the prepared benzene solution into the constant temperature bath which should be kept at 25 C (the flasks should be equipped with hooks which can fit into a set of holes drilled around the lid of the temperature bath). The procedures are described in S&G but the capacitance cell used (see Fig. 1) is a home-made variation of the one described in the text book. Figure 1. Air capacitor cell arrangement used for the measurement of dipole moment of liquid compounds. The cell can be filled or emptied through the side arm without exposing the fragile air capacitor blades. The value of the capacitance may be adjusted by rotating the knurled knob at the top; electrical connections are made through a BNC connector. A portion of the solution to be tested (enough to cover the plates of the air capacitor) is transferred to the capacitance cell, which is clamped and kept partially immersed in the temperature bath during measurement.

3 Chem Page VII - 3 The value of the air capacitor immersed into air, then into each of the prepared sample solution is to be measured in two positions, closed and open ; repeat the measurements, alternating between each position at least three times. The refractive indices of all solutions should also be measured. Explain clearly in your lab write-up why this should be done (Discussion section). As soon as all your solutions are prepared, you may want to secure a sample 1 ml of each solution into separate stoppered vials which are put aside for the refractive index measurements or make sure that some solution is left in each volumetric flask. 5. Data Analysis and Discussion The data are most conveniently processed with the help of a spreadsheet in combination with a [weighted] linear least-squares fitting program which also outputs the errors on slope and intercept (Do not forget to include the pure benzene measurements in the dataset). Include in the Results section of your report a table of the raw data in the format shown below. Be aware that with the set-up used in the lab, one measures capacitance, not frequencies as in S&G. Consequently, the dielectric constant κ, is simply calculated using Eq. (26) in S&G. When you turn in your report, submit also your raw data (unprocessed) on a diskette (or as an to the instructor and TA) by filling in the MS-Excel spreadsheet dipole_moment_results.xls (available from the course web site) in the following format: Sample Mass empty vial Mass vial + solute Mass + solvent Capacitance position 1 position 2 Index of refraction Benzene nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn o-dicl 1 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn o-dicl 2 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn o-dicl 3 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn o-dicl 4 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn m-dicl 1 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn m-dicl 2 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn m-dicl 3 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn m-dicl 4 nn.nnnn nn.nnnn nn.nnnn nnn.nn nnn.nn n.nnnn Air nnn.nn nnn.nn - Answer the questions from the text book. Why it is important to know the dipole moment of a molecule?

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5 Chem Page VII - 5 Chem 366W report check list A report will not be accepted without all the items of this list checked. If a checked item is found missing in the report, the report will be automatically down-graded. Student Name: Report: Dipole Moment of Polar Molecule in Solution Title page. Correct title of the experiment... Student Name & student ID... Partner name (if applicable) Date of performance of experiment... Abstract... Introduction and theory... Experimental Changes from text description mentioned (if applicable)... Sample ID, ser no, stock solution etc recorded (if applicable)... Results Results as Tables... Graphs Size, at least ½ page... Axis labelled... Axis labels have units... Axis scales are sensible... Only significant figures... Uncertainties quoted... Raw data provided (electronic form, if applicable)... Calculations Sample calculation provided... Error analysis... Sample error calculation provided... Discussion Comments on results... Questions in text book and in manual answered... Comparison with literature value(s)... Conclusion...

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