Posted: March 24th, 2017
3. Calculate the pZn of a solution prepared by mixing 25.0 mL of 0.0100 M EDTA with 50.0 mL of 0.00500 M Zn2+. Assume that both the Zn2+ and EDTA solutions are buffered with 0.100 M NH3 and 0.176 M NH4Cl. 4. Calculate the pH resulting from mixing 15.00 mL of 0.800 M HIO3 (pKa = 0.77) with 45.00 mL of 0.0200 M NaOH. 6. Consider the electrochemical cell described below.(????) 7. A 3.67 g sample of insecticide was decomposed in acid, any As5+ was reduced to As3+ and diluted to 250.0 mL in a volumetric flask. A 5.00 mL aliquot was added to 125.0 mL of 0.0500 M KI buffered to pH 7. A coulometric titration was carried out with electrically-generated I3-, which oxidized As3+ to As5+ according to the reaction. As3+ + I3-? 3I- + As5+ The titration required 287 s at a constant current of 24.25 mA to reach the endpoint. Calculate the percentage of As2O3 (197.84 g/mol) in the insecticide. 8. A 2.05 g sample of iodized salt was dissolved in water and diluted to 50.0 mL. Measurement with an iodide ISE gave a value of + 62.5 mV. Then 5.00 mL of a standard solution containing 2.50 x 10-4 M KI was added and the potential reading was + 42.7 mV. Calculate the percentage of KI (166.00 g/mol) in the salt sample. Calculate the molar concentration of K in the measured solution as follows: 11. Butanoic acid has a partition coefficient of 3.0 (favoring benzene) when distributed between water and benzene. Find the formal concentration of butanoic acid in each phase when 100.0 mL of 0.10 M aqueous butanoic acid is extracted with 25.0 mL of benzene at a pH of 4.00. 12. (15 pts.) A 480 mg sample of cat food is digested with HNO3 and HClO4 and diluted to 25.0 mL in a volumetric flask. Three aliquots of 2.00 mL each are drawn, to which are added 0.1 mL, 0.2 mL and 0.3 mL spikes of 100.0 ppm Fe standard solution. The absorbance of an unspiked 2.00 mL aliquot is measured by AAS, and the absorbance of the spiked samples is measured as well. The data is presented in the Table below. Correct the absorbance of the spiked samples for dilution and plot the absorbance versus ppm Fe added. Extrapolate the graph to the point where absorbance is equal to zero to determine the amount of Fe in the cat food sample in ppm. Sample A Original 0.186 + 0.1 mL 0.295 + 0.2 mL 0.391 + 0.3 mL 0.478
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