exp29-d8-6-07 - Experiment 29 Physical Chemistry Laboratory...

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1 Experiment 29 Physical Chemistry Laboratory Dipole Moment of Polar Molecules in Solution (revised 6//07) Introduction The object of this experiment is to determine the dipole moment of a polar compound (acetophenone or methyl benzoate) from measurements of the densities and dielectric constants of dilute solutions of the compound in cyclohexane. From this data and the refractive index of the polar compound, the dipole moment of these is obtained. Theory Be sure to read a relevant discussion of dipoles, refractive index and polarizability of molecules. *It is suggested to preserve reagents, the order of performance of this experiment should be:
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2 1. dielectric determination, 2.density, and lastly 3. refractive index. Procedure Density Solutions of 1.0, 2.0, 3.0, 4.0, 5.0 and 7.0 mole percent of the polar compound in cyclohexane are provided. The densities of these solutions, and of pure cyclohexane and acetophenone are to be determined with a pycnometer . Pycnometers are volume-calibrated containers used to determine the density of liquids or solids. They are particularly convenient for determining the density of liquids. Fig.1 picnometer Calibration: Calibrate the pycnometer (accurately determine its volume) using distilled water as the standard (V = m/ ρ ). Use a CRC to get the density of distilled water at its measured temperature. Average at least three volume determinations. General Procedure: Record the mass of the empty pycnometer with the capillary stopper and thermometer in place. Fill the pycnometer with the liquid of interest and then place the thermometer and the capillary stopper in place; some of the liquid should overflow from the capillary. Using a Kimwipe, dry the outside of the pycnometer and then wipe the top of the capillary stopper clean. Record the mass and temperature of the filled pycnometer. To obtain the mass of the liquid, subtract the mass of the empty pycnometer from the mass of the filled pycnometer. Use the calibrated volume of the pycnometer and the mass of the liquid to determine the density of the liquid. It is important to thoroughly dry the picnometer when determining the empty volume. Use an aspirator and/or dry with acetone. Note: It is important to minimize the time it takes to measure the mass of the liquid-filled pycnometer. Consider the effects of evaporation from the capillary and the influence of your body's temperature on the liquid's temperature. Refractive index Fig. 2 Abbe refractometer The refractive index of either acetophenone or methyl benzoate is determined with an Abbe refractometer using light from a sodium vapor lamp. The Abbe refractometer is described on p.659 (Chapter XX) of Shoemaker, Garland, and Nibler, 6 th ed. [1] (see attachment). The refractive index should be measured for all molar solutions along with the pure liquids (e.g. cyclohexane and acetophenone).
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exp29-d8-6-07 - Experiment 29 Physical Chemistry Laboratory...

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