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Exp27 - Nick Benzschawel Sarah Weinreis Chemistry 563 Sec 6...

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1 Nick Benzschawel Sarah Weinreis Chemistry 563, Sec 6 3/20/09 Experiment 27 Conductance Behavior of Strong Electrolytes Abstract Resistance measurements of a standard solution of 0.200 M KCl were made in order to determine the cell constant, which was 117.69 0.07 m -1 . Next, the cell constant in combination with conductivity data obtained from solutions of strong electrolytes were used to calculate specific conductances, which were then used to determine equivalent conductances. From a plot of equivalent conductance versus square root of concentration the B value in Onsager’s equation was determined to be 2542.29 -1 ·cm 7/2 ·eq -3/2 . This plot also allowed the determination of the equivalent conductance at infinite dilution (414.47 ± 0.97 S·cm 2 ·eq -1 ). Next, the moving boundary method was used for determination of the transference numbers for H + and Cl - , which were 1.4822 ± 0.0166 and -0.4822, respectively. Finally, the combined data from both experiments (e.g. transference numbers and equivalent conductance) were used to find the ionic mobilities for these two ions, which were 7 10 ) 0740 . 0 ( 912 . 5 m 2 ·V -1 ·s -1 and 7 10 ) 067 . 0 ( 947 . 1 m 2 ·V -1 ·s -1 , respectively. Introduction The purpose of this experiment was to use measurements of the resistance of solutions of strong electrolytes in combination with data obtained from the moving boundary process in order to determine ion mobilities. Resistance measurements of a standard KCl solution were made using YSI 3200 Conductivity Instrument in order to determine the cell constant, k cell , by the equation: cell k κ R 1 (1) where R is the resistance and is the specific conductance. Next, resistance measurements of HCl solutions ranging from 0.1-0.001M were made in order to determine specific conductances of each of these solutions. With k cell already determined these calculations were easily made using equation (1). With specific conductances calculated, these values were used to determine equivalent conductances using the equation: c κ Λ (2) where c is the concentration of a particular HCl solution and is the equivalent conductance. With equivalent conductance data at hand, and in combination the square root of concentration, the equivalent conductance at infinite dilution, o Λ , and the value for B in Onsager’s equation:
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2 1/2 o Bc Λ Λ (3) could be determined from a plot of equivalent conductance versus square root of concentration. The second portion of the experiment focused on determining transference numbers for H + and Cl - . To do this an approximately 0.1 M HCl solution was concocted and utilized in the moving boundary apparatus. This apparatus allowed easy determination of the change in volume with time (dV/dt) because it was graduated every 0.1 mL. Current, time, volume and concentration data, along with Faraday’s Constant were all utilized to determine the transference number of the cation (T c ) through the equation: I Fc(dV/dt) T c (4) Through the relationship, T c +T a =1 the transference number for the anion was also determined.
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