A Sample Exam
This examination is given under the same pledge conditions as if it were taken in a scheduled 90 minute class period, in a scheduled classroom. IT IS NOT A "Take-Home" EXAM.
1. The entire session for this exam, which consists of eight questions, is timed at 90 (ninety) minutes maximum.
2. You may work on the exam in any location you choose, and over any 90 minute time interval you wish, between Thursday, 10/23/97 and the start of class on Monday, 10/27/97. Work on the exam in one block of time, with only the same kind of interruptions you would need were you taking it in a classroom.
3. You are to do your work alone, under closed book conditions, without discussion with, or assistance from, other people, and using no external resources other than your calculator and the material provided in this envelope.
4. You are not to discuss the exam with others until after all members of the class have handed in their examinations.
5. Include this pledge, signed or not, in this envelope, sealed, with your exam when you turn it in.
6. Do not open this envelope until you are ready to work on and complete the exam. You will keep track of your own time, and stop working on the exam at the 90 minute limit mentioned in (1.) above.
Signing the PLEDGE statement below signifies that you have agreed to, and abided by, all of the above conditions.
I pledge my honor that during this examination I have neither given nor received assistance and that I have seen no dishonest work.
Please Print your Name: _____________________________
(1.) (15 Points)
Using the potential-time "excitation" waveform shown below, sketch on the graph provided what the voltammogram taken with it vs. SCE using a Static Mercury Dropping Electrode would look like. Assume that the Faradaic reaction is:
Ox + e- <==> Red
Also, explain what the symbols S1 and S2 on the potential-time waveform below represent.
(2.) (10 Points)
A "device" is shown in the figures below that we have not discussed in class but is presented in your reading assignments for this examination. It is used for the analysis of very small amounts of liquids for trace metals by atomic absorption. Answer the following questions about this device:
(a.) What is it called?
(b.) Show in inset A the light path through the device and the analyzing monochromator.
(b.) Show in insets B and C where the analytical sample must be placed before the device is operated
(3.) (10 Points)
Shown below is a block diagram and a drawing of a device taken from your book. The processes labeled in the block diagram all occur to some degree in the device.
(a.) What is the complete technical name for the entire device?
(b.) Air and acetylene are two gases that are used in this device. Label where they enter the device.
(c.) The analytical sample is a liquid. Label where it enters the device.
(d.) Mark where the analytical signal is generated by the device.
(e.) Then mark the place or places where the process called "encrustation" could occur. Explain briefly how this process could effect the analytical accuracy of the device.
(4.) (10 Points)
Shown on this page are optical diagrams of two completely different instruments that are used to accomplish the same analytical goal.
(a.) What is that analytical goal?
(b.) What are two fundamental differences between the two instruments?
(c.) Locate and give the correct technical name for the detectors in both instruments.
(d.) Circle the diffraction grating in both instruments. Indicate which instrument uses a plane grating, and which a concave grating.
(5.) (15 Points)
Shown above is an apparatus that we all used in the lab. It combined all of the technology mastered in the first three experiments into one practical problem solving project.
Briefly, (a.), state what the central objective of this project was, (b.), identify and explain the function of the five numbered parts of this apparatus shown in the figure, and (c.), explain what the three technologies were that were combined in the project done with it.
While this is not an essay question as such, clear organization of your answer is important.
(6.) (20 Points)
Shown above is the atomic absorption instrument. The instrument uses an air/acetylene flame as an absorption cell. The flame is quite bright.
As indicated in the figure, the monochromator used in the instrument occupies only a small part of it. Since the whole instrument is only 3 feet or so long, and since at least half of it is taken up with the hollow cathode lamp and the burner, it is clear that the focal length of the mirrors in the monochrometer must be on the order of 0.25 meter. This means that the monochromator has a low dispersion, likely on the order of 5 to 10 nanometers/millimeter, and correspondingly low resolution (i.e., a wide bandpass).
(a.) How can a low resolution monochromator with a wide bandpass be used to measure small changes in concentration, given that the absorption lines are so narrow in a flame, without such severe working curve nonlinearity that the techniques is worthless?
b.) How can any absorption be reliably measured at all in a flame that is itself emitting light at the same wavelengths that the absorption is supposed to be occurring?
(7.) (10 Points)
In the figure above, a "knob" is shown, which when rotated causes the "wavelength presented to the exit slit" of the Czerny-Turner monochromator inside the instrument shown in question 6 to change.
A sketch of the "sine-bar" device to which the knob is connected is also shown below.
(a.) Indicate on the sine bar sketch to what the knob must be connected to make the wavelength at the exit slit change when the knob is rotated.
(b.) Why is the device that causes the wavelength to change called a "sine bar"?
(c.) On the next page, sketch a Czerny-Turner monochromator, showing the knob, the sine bar device, and any other parts appropriate to make the wavelength change at the exit slit when the knob is rotated.
(d.) Although the Czerny-Turner configuration for a monochromator is old, there is at least one reason why it is used in modern instruments such as that shown in question 6. What is that reason?
(8.) (10 points)
The instrument shown above uses a light emitting device that we discussed in class, and that also is presented in the reading assignment for this exam in your book.
(a.) What is this device called? Circle its location in the instrument.
(b.) What makes this device so different from the air/acetylene flame that is used in the atomic absorption experiment?
(c.) Circle the location of the analytical sample in the above diagram.
(d.) What is the function of the "radio frequency generator"?
(e.) The spectrometer shown in the above diagram is quite different than the Czerny-Turner monochromator used in the atomic absorption experiment. Why is this?