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exam 1 solution - Name Prof.RyanC.Bailey...

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Name: _______________________________ Chemistry 420 – Fall 2008 Prof. Ryan C. Bailey Exam #1 (175 points total) This exam is 8 pages long Due at the end of class Please box all answers Show all of your work for each answer. Partial credit is awarded based upon the work that you show. We can’t give credit if we don’t know what you did. Be careful to use correct significant figures and propagate errors through mathematical operations, when appropriate. Recommendation : Read the entire exam first and come up with a strategy to maximize your accumulated points. You do not have to work the problems sequentially. Key
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Chem 420 – Fall 2008 – Exam #1 Name: _______________________________ Page 2 of 8 Appetizers—Responses for questions 1 4 can be answered in just a few sentences 1. (15 points) How does slit width impact an optical measurement made with a grating monochromator? In general, what is the optimal slit width? (Note: I am not looking for a numerical answer, but rather the general objective) Decreasing the slit width of a monochromator increases the resolution (decreases the spectral bandwidth) (5 pts) Decreases the slit width also reduces the light throughput (5pts) In general, one should use the largest slit width capable of providing the required resolution for the measurement (5 pts) 2. (10 points) In atomic emission spectroscopy, why is an inductively coupled plasma source less susceptible to interfering chemical species and matrix effects than a flame source? The plasma is significantly hotter (2 3 times) than the highest temperature fuel oxidant flame. At these higher temperatures, atomization is much more complete (7 pts) and thus there is a reduction in interference and matrix effects. Additionally, the inert environment of the plasma (no oxidant) prevents oxide formation (3 pts), which in turn increases the lifetime of the element. Key
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Chem 420 – Fall 2008 – Exam #1 Name: _______________________________ Page 3 of 8 3. (15 points) Atomic line widths are broadened by four effects: the uncertainty effect, the Doppler effect, pressure broadening, and electric and magnetic field effects (not discussed in class). Pick two of the three we discussed and briefly explain their origin. Of the two you choose, which has the largest contribution to line broadening? Uncertainty : The finite (and short) lifetime of the transition states leads to uncertainty in the spectral frequency. 1 t ν Δ Δ ≥ Doppler: The wavelength of radiation emitted or absorbed b y a rapidly moving object decreases or increases as the object moves toward or away from the transducer . Atoms in these measurements are moving.
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