Chemistry 421/821 – Final Exam Spring 2010 page 1Name ANSWER KEY

SECTION I: Voltammetry, MS and NMR (100 points)

1)(12 points)Given the following voltammogram:

  1. (3 points)What is the significance of the midpoint (Y)?

It is related to the standard potential (Eo) of the half-reaction.

  1. (3 points)What would happen to the plot if the concentration doubled?

The height of the graph would double or the maximum current would be 160 A.

  1. (3 points)What experimental parameter is changing as a function of time to collect the voltammogram?

Applied potential

  1. (3 points) Why is there a lack of current at the beginning of this curve?

The applied potential has not reached the potential required for the half-reaction to occur.

2)(15 points) Determine the structure for compound: C9H12O. NMR Solvent is CDCl3. Relative intensities (integral) of 1H NMR integrals and splitting patterns are indicated.

3)(10 points) Draw a stick diagram indicating the peak splitting for each hydrogen atom labeled A. Assume the coupling constants,if they exist, between A and 1 is 3 Hz and between A and 2 is 6 Hz.

  1. (5 points)

Each coupling splits the peak into a doublet, a doublet of doublets

  1. (5 points)

No splitting from 1, too many bonds away. The peak is split into a quartet, 3 hydrogens on the methyl group for 2

4)(20 points) Please answer the following general questions about NMR.

  1. (4 points) Identify one approach to increase the sensitivity of an NMR experiment.

Increase the magnetic field strength, increase the concentration, increase the number of scans, decrease the temperature.

  1. (4 points) What chemical feature or structural characteristic is required to observe, between two nuclei, an:
  2. NOE

Nuclei have to be within a 6Åthrough space separation

  1. a coupling constant

Nuclei have to be connected through ~ 1-3 bonds.

  1. (4 points) In general, what causes different chemical shifts for different nuclei?

Different local magnetic fields caused by different electron density.

  1. (4 points)What is meant by a 90o pulse?

The net magnetization along the Z-axis is flipped into the X,Y-plane (90o) by the application of an RF pulse of the correct duration.

  1. (4 points) What is T1 relaxation?

The relaxation of the net magnetization in the X,Y plane back to the being aligned along the positive Z-axis by exchange of energy with the solvent.

5)(15 points)Given the following two structures:

Isotope / Symbol / Atomic Mass / Percent Abundance
1 / H / 1.007825032 / 100
2 / H / 2.014101778 / 0.0115
12 / C / 12.0000000 / 100
13 / C / 13.0033548378 / 1.08
35 / Cl / 34.96885271 / 100
37 / Cl / 36.96590260 / 32.5
79 / Br / 78.9183376 / 100
81 / Br / 80.916291 / 97.3
  1. (4 points) What is the
  2. (2 points)Nominal mass for compound 1.

12 x 7 + 1 x 7 + 79 x 1 = 170

  1. (2 points)Exact mass for compound 2.

12 x 7 + 1.007825032 x 7 + 34.96885271 x 1 =126.0236279

  1. (5 points) You are using an inexpensive mass spectrometer that only has a resolution of 500. Is that sufficient to determine if both compounds 1 and 2 are in an unknown mixture? Please show all work and explain.

Easily since the needed resolution is much smaller than the instrument resolution.

  1. (6 points) Is the M+4/M ratio for compound 1 more or less intense than the M+2/M ratio for compound 2? Please show all work.

The most probably M+4 for compound 1 is two 81Br = (97.3/100)2 = 0.95

The most probable M+2 for compound 1 is one 37Cl = 32.5/100 = 0.325

Thus, the M+4/M ratio for compound 1 is more intense than the M+2/M ratio for compound 2

6)(20 points) Please answer the following general questions about MS:

  1. (5 points) Whatis the important difference between hard ionization and soft ionization?

Hard ionization causes molecular fragmentation, soft ionization doesn’t.

  1. (5 points)Give one reason why mass spectrometers work under a high vacuum.

Minimizes collisions that either neutralizes the molecule or interferes with its trajectory an apparent m/z ratio.

  1. (5 points) Why does a molecule have to been ionized to be analyzed by a mass spectrometer?

The trajectory or speed of an ion through electric field and magnetic field depends on its mass and charge. Thus, if is not charged it can’t be detected.

  1. (5 points) Does a mass spectrometer directly measure mass?

No, m/z ratio

7)(8 points) What is the source of ionization for the following MS techniques (Be Specific!):

  1. (2 points) Electron-Impact Source (EI)

Electron beam

  1. (2 points) Chemical Ionization Source (CI)

Beam of methane ions

  1. (2 points) Fast Atom Bombardment Source (FAB)

Beam of high energy xenon, argon atoms or cesium ions

  1. (2 points) Electrospray Ionization (ESI)

Electrode at a high potential

SECTION II: Review (100 points)

8)(20 points) Please answer the following general questions about instrumental analysis:

  1. (5 points)Please identify one potential source of systematic error specifically related to the operation of an amperostatic (coulmetric titration).

Departure from 100% current efficiency

  1. (5 points)Please identify one potential source of random error specifically related to the operation of UV-vis spectrometer.

Shot noise, flicker noise

  1. (5 points)What is the difference between precision and accuracy?

Precision is the reproducibility of a measurement and accuracy is how close the measurement is to the “true” value.

  1. (5 points) What is difference between sensitivity and selectivity? (Hint: I am looking for the technical/mathematical definition.)

Sensitivity is the slope of the calibration curve, while selectivity is the ratio of two slopes for two different analytes.

9)(15 points) Please answer ONLY ONE of the two following questions:

  1. Please describe a process that leads to line broadening in atomic spectroscopy and why it is not important in other spectroscopy techniques.

Uncertainty principal - t .n ≥1peaks have a finite width because of the uncertainty in simultaneously measuring both frequency and time.

Doppler effect – emitted or absorbed wavelengths changes as a result of atom movement relative to detector. Wavelength decrease if motion toward receiver, wavelength increases if motion away from receiver.

Pressure broadening - Collisions with atoms/molecules transfers small quantities of vibrational energy (heat) - ill-defined ground state energy. Energy states are randomized.

These issues are uniquely important to atomic spectroscopy because of the extremely narrow/sharp line-widths to begin with. So, the small changes caused by these factors become significant, whereas; in other spectroscopic techniques the lines are relatively broad and these effects are inconsequential. Atomic spectroscopy lines are sharp because there are no superpositions of vibration energy levels with the electronic absorption that are present in all other techniques. Atoms don’t have bonds, no bonds – no vibrations.

  1. Please use an energy diagram to explain any fluorescence deactivation process.

Vibrational relaxationinternal conversion intersystem crossing

.

10)(15 points) Please answer ONLY ONE of the two following questions:

  1. Please describe four different types of vibrations found in a molecule.

  1. Numerous times throughout the semesters we have referred to the Boltzmann equation:

to understand some aspect about the operation of an instrument. What does the Boltzmann equation tell us and why is this important?

The Boltzmann equation tells us the population ratio between two different energy states as a function of the energy difference between those two states and the system’s temperature. The Boltzmann equation is important to instrumental analysis because the population difference is directly related to the instruments sensitivity. The higher the energy gap and population difference the more sensitive the device because there are more particles (atoms, molecules, electrons, nuclei) that can be promoted from the ground state to the excited state. The Boltzmann equation explains why NMR is relatively insensitive and fluorescence spectroscopy is relatively sensitive.

11)(20 points) Please answer the following general questions about Chromatography.

  1. (8 points) The Van Deemter equation: H = A + B/ + C equates plate height (H) to flow rate () to determine the optimal conditions for the chromatographic separation.
  2. (4 points) In general, what do the constants A, B, C represent?

Various factors that contribute to peak broadening.

  1. (4 points) How is a plot of the Van Deemter equation used to determine the optimal operational condition?

The plot minimum indicates the optimal flow rate that maximizes performance (number of plates or minimal plate height).

  1. (5 points) In capillary electrophoresis why do all solutes, regardless of charge, migrate to the negative electrode of the system?

Electroosmotic flow - net positive charge of solution, exists to counter negative charge of capillary surface, causes net migration to the negative electrode.

  1. (5 points) In gas chromatography, the mobile phase or carrier gas only transports the solute through the column and is not involved in the separation. What two factors determine a solute’s separation in gas chromatography?

Equilibrium is between analytes affinity to stationary phase and solute’s volatility.

  1. (2 points) What is a common carrier gas in gas chromatography?

Helium

12)(20 points) Please answer the following general questions about Electrochemistry.

  1. (5 points) For the following electrochemical cell, please label the anode, cathode, salt bridge, the species that is oxidized and the species that is reduced.

Hg|Hg22+(4.00x10-2 M)||H+(3.00x10-2M),V3+ (2.00x10-2M), VO2+(6.00x10-3M)|Pt

  1. (5 points) What is the cause of a liquid junction potential?

Ions of different size diffuse at different rates across the junction causing a separation in charge and a corresponding potential.

  1. (5 points) Why is the presence of a current in a potentiometric measurement a source of error?

The goal is to measure the potential associated with a specific half-reaction that is related to the concentration of the analyte of interest. Any current present in the cell will also cause an additional potential according to Ohm’s law (E=IR). So, in the presence of a current the observed potential will be in error.

  1. (5 points) What is an advantage of pulse voltammetry?

Experimentally generates a first-order derivative of thevoltammogram that improves the accuracy in measuring the peak current (concentration) and the inflection point, now peak height, which correlates to the standard potential of the half-reaction.

13)(10 points) Please answer ONLY ONE of the two following questions:

  1. Some light sources used for UV-vis and IR spectrometers work by the same principal or mechanism. Please describe this common mechanism and provide an example light source.

Black body radiation – heat a solid filament to glowing, where light emitted is characteristic of the temperature and properties of the solid.

Examples: tungsten filament lamp, Nernst glower, globar, incandescent wire source.

  1. UV-vis spectroscopy, fluorescence spectroscopy, atomic absorption spectroscopy, and IR spectroscopy routinely utilize a double beam spectrometer. Why?

These spectroscopic techniques measure a transmittance (T= P/Po), which is the ratio of the power of the residual light (P) (after passing through sample) and the source light (Po). Thus, two measurements of light intensity or power are necessary. A dual beam spectrometer allows for making both measurements essentially simultaneous instead of sequential. Thus, a dual beam instrument significantly improves the accuracy by eliminating/minimizing common sources of error.