Chemistry 325 Instrumental Methods of Analysis March 13, Final Exam. Name

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1 Final Exam Name Instructions: This exam is worth 100 points. Some questions allow a choice as to which parts are answered. Only answer the number of parts requested.

2 1. (32 points) Circle the best answer for each of the following. a. In an electrochemical cell, (1) electrons flow from the cathode to the anode. (2) the salt bridge allows charge to equilibrate between the two half-cells. (3) oxidation takes place at the cathode. (4) None of the above. b. Electrodes of the second kind (1) are based on a half cell consisting of a metal, a slightly soluble salt of the metal, and the anion of the salt. (2) have a potential that is controlled by the activity of a species not involved in the redox reaction. (3) are commonly used with ion selective electrodes. (4) All of the above. c. Standard reduction potentials (1) are based on assigning the saturated calomel electrode a potential of 0.0 volts. (2) are based on the assumption that all reactants and products in the half-cell reaction are in their standard states. (3) greater than zero indicate the occurrence of a spontaneous oxidation reaction. (4) None of the above. d. Which of the following statements is false? (1) The junction potential derives from an unequal charge distribution on the two sides of a boundary. (2) The junction potential established across a charge-selective membrane serves as the basis for ion selective electrodes. (3) In a glass ph electrode, the junction potential forms due to the presence of holes in the glass membrane that are sized specifically for hydrogen ions. (4) The junction potential in an ion selective electrode is related to the logarithm of the analyte ion activity. e. In a chromatography experiment, (1) the mobile phase flow rate has no effect on retention time. (2) increasing the length of the stationary phase has no effect on the capacity factor. (3) HETP has no effect on resolution. (4) The number of theoretical plates has no effect on peak width. f. Quantitative analysis in gas chromatography (1) is sensitive to variation in the amount of sample injected. (2) often requires the use of an internal standard. (3) requires calibration standards to measure response factors. (4) All of the above.

3 g. Gas chromatography (1) is based on electrostatic interactions between the analyte and stationary phase. (2) requires samples that can be decomposed by heating. (3) can employ detection based on the thermal conductivity of the column effluent. (4) often employs a procedure in which the mobile phase composition is changed during the experiment. h. In a voltammetry experiment, (1) three electrodes are required in order to compensate for the potential on the working electrode changing as the reaction proceeds. (2) the potential on the working electrode is recorded and plotted vs. analyte concentration. (3) the working electrode is constructed from the metal of the analyte ion. (4) the half-wave potential signals the completion of the redox reaction. 2. (40 points) Answer five of the following. a. Describe the membrane used in a fluoride ion selective electrode. b. In a voltammetry experiment conducted in an unstirred solution, a sharp current peak is observed, followed by a slow reduction in current until a steady-state or "limiting" current is reached. Describe the origin of this current profile. c. Define the term, "chromatographic efficiency". Why are capillary columns in gas chromatography more efficient than packed columns? d. Use the Nernst equation to show that an Ag/AgCl electrode is an "electrode of the second kind".

4 e. Describe the flame ionization detector in gas chromatography. f. In chromatography, why is the capacity factor used to describe the retention of an analyte by the stationary phase rather than the retention time? 3. (8 points) Compute the potential of the following electrochemical cell: Pt Ti 2+ (0.1 M), Ti 3+ (0.01 M) MnO 4 - (0.01 M), Mn 2+ (0.1 M), H + (ph 2.0) Pt 4. (10 points) A fluoride ion selective electrode is calibrated over the range of to 0.01 M and found to have an intercept of mv and a slope of mv/decade. A water sample containing an unknown fluoride concentration is analyzed by taking a 2.00 ml sample of the water and diluting it to ml in a volumetric flask. Employing the calibrated electrode, the potential of the resulting solution is measured as mv. What is the concentration (in M) of fluoride in the original water sample?

5 5. (10 points) You are employing a 10 m gas chromatography column with an HETP of m/plate. Given this column, compute the resolution that would be obtained between two peaks whose retention times are 1.21 and 1.33 minutes, respectively.

2. Which of the following statements best describes the movement of electrons in an electrochemical cell?

2. Which of the following statements best describes the movement of electrons in an electrochemical cell? Exam 2 Chem 311 Evans Fall 2009 112: 2 pts each 1. Consider the following unbalanced redox equation: Pb (s) + PbO 2 (s) + 2 HSO 4 (aq) 2 PbSO 4 (s) Which species is being oxidized? A. HSO 4 B. Pb(s) C.