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Rules: You may use the following materials: (1) your textbook; (2) your class notes; and (3) any downloaded items from the course website. Please do your own work, no collaborating. Work the problems on the exam sheet and show all of your work. Please hand in both your answers and the exam sheet. Good luck! Due: No later than 1 pm Friday, September 13, 2013. Slide your exam under the door of Prof. Buratto’s office (4148B Chemistry) or hand it to Prof. Buratto himself. 3. (20 pts.) 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. (15 pts.) 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. (15 pts.) Consider the electrochemical cell described below. ????????(????)|????????????????(????)|????????????(????????, ????????????????????????????????????)||???????????????? ????????????????????????????????|????????(????) The cell solution was made by mixing 25.0 mL of 4.00 mM ???????????? 25.0 mL of 4.00 mM ????????????(????????)? 25.0 mL of 0.400 M acid, ????????, with pKa = 9.50 25.0 mL of ???????????? solution The measured voltage was -0.440 V. Calculate the molarity of the KOH solution. Assume that essentially all of the copper(I) is in the form ????????(????????)? ?. For the right side (the cathode), the half cell reaction is: ????????(????????)? ? +????? ? ????????(????)+2????????? ????° = ?0.429 ???? 7. (20 pts.) 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. (10 pts.) 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. 9. (20 pts.) The sulfur in a 5.00 g sample of steel was evolved as H2S and then collected in a solution of CdCl2 to produce the precipitate CdS. The CdS was then titrated with excess I2 and the remaining I2 was then back titrated with 4.82 ml of 0.0510 M sodium thiosulfate. If the concentration of the I2 is 0.0600 M and a total of 10.0 ml is added, calculate the %S in the steel. The relevant reactions are: CdS + I2 ? S + Cd2+ + 2I- Titration (excess I2) I2 + 2S2O3 2- ? S2O6 2- + 2I- Back Titration 10. (15 pts.) Inorganic phosphorous is spectrophotochemically measured in blood serum by formation of a reduced heteropoly acid (heteropoly blue) and comparison of its absorbance with a standard treated by the same procedure. If a 1.00 mL blood serum sample gives an absorbance of 0.217, while a 2.00 mL aliquot of a standard containing 91.2 mg KH2PO4 per liter gives an absorbance of 0.285, calculate the milligram percent of P (milligrams per 100.0 mL) in the blood sample. Note: both aliquots are diluted to the same volume prior to absorbance measurements. 11. (15 pts.) 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

Rules: You may use the following materials: (1) your textbook; (2) your class notes; and (3) any downloaded items from the course website. Please do your own work, no collaborating. Work the problems on the exam sheet and show all of your work. Please hand in both your answers and the exam sheet. Good luck!

Due: No later than 1 pm Friday, September 13, 2013. Slide your exam under the door of Prof. Buratto’s office (4148B Chemistry) or hand it to Prof. Buratto himself.

3. (20 pts.) 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. (15 pts.) 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. (15 pts.) Consider the electrochemical cell described below.

????????(????)|????????????????(????)|????????????(????????, ????????????????????????????????????)||???????????????? ????????????????????????????????|????????(????)

The cell solution was made by mixing
25.0 mL of 4.00 mM ????????????
25.0 mL of 4.00 mM ????????????(????????)?
25.0 mL of 0.400 M acid, ????????, with pKa = 9.50
25.0 mL of ???????????? solution
The measured voltage was -0.440 V. Calculate the molarity of the KOH solution.
Assume that essentially all of the copper(I) is in the form ????????(????????)?
?. For the right side
(the cathode), the half cell reaction is:

????????(????????)?
? +????? ? ????????(????)+2????????? ????° = ?0.429 ???? 7. (20 pts.) 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. (10 pts.) 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. 9. (20 pts.) The sulfur in a 5.00 g sample of steel was evolved as H2S and then collected
in a solution of CdCl2 to produce the precipitate CdS. The CdS was then titrated with
excess I2 and the remaining I2 was then back titrated with 4.82 ml of 0.0510 M
sodium thiosulfate. If the concentration of the I2 is 0.0600 M and a total of 10.0 ml is
added, calculate the %S in the steel. The relevant reactions are:

CdS + I2 ? S + Cd2+ + 2I-
Titration (excess I2)

I2 + 2S2O3
2-
? S2O6
2-
+ 2I-
Back Titration 10. (15 pts.) Inorganic phosphorous is spectrophotochemically measured in blood serum
by formation of a reduced heteropoly acid (heteropoly blue) and comparison of its absorbance with a standard treated by the same procedure. If a 1.00 mL blood serum sample gives an absorbance of 0.217, while a 2.00 mL aliquot of a standard containing 91.2 mg KH2PO4 per liter gives an absorbance of 0.285, calculate the milligram percent of P (milligrams per 100.0 mL) in the blood sample. Note: both aliquots are diluted to the same volume prior to absorbance measurements.
11. (15 pts.) 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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