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White light strikes a grating and is dispersed into a spectrum. In the figure the spectrum appears spread out from A to B and we also see the spectrum repeating in second order. Which description applies? (select one)Final Exam
May 9, 2002
(One figure in I-10 was omitted. Other figures may move to odd locations on the Web page version of the exam)
Part I-- Short Answers (Answer all questions in Part I-- 10 questions 5 points each = 50 points)
1. light energy... a typical wavelength in the near IR region is _ <800-2500> _ nm
... a wavelength of 1.45 nm is in the _ X-ray_region of the spectrum.
2. .
White light strikes a grating and is dispersed into a spectrum. In the figure the spectrum appears spread out from A to B and we also see the spectrum repeating in second order. Which description applies? (select one)
___A(red) to B(violet) and the second order appears around location c
___A(red) to B(violet) and the second order appears around location d
___A(violet) to B(red) and the second order appears around location c
_X_A(violet) to B(red) and the second order appears around location d
formula says n(lambda)=d (sin theta + sin gamma) where sine theta is fixed. Therefore longer wavelength match large sines or larger angles. Thus B is red and A is violet. When n=2 the sine again must be larger so second order is at d.
3. The refractive index of a crystal is defined as... .the ratio of the speed of light in vaccum / speed of light in the medium (the crystal)
4. Five determinations gave a value of 34.213 + 0.022 grams. How many additional measurements are needed to get the measurement to + 0.01 g. (Assuming, of course, that all error is statistical / random.) Answer =_15 or 19__ more measurements.
An approximate answer is a reduction of noise by a factor of 2. Since S/N ratio is protortional to sqare root of N we need four times as many measurements (20 total or 15 more.) If you are a purist we really need (0.022/0.01)2 = 4.84 times or a total of 24.2 measurements meaning we need 19 more.
5. 
A signal at 12.0 volts is applied across a series of resistors.
This is a voltage divider with 4K out of a total of 32K-- thus we have a voltage drop of (4/32)x12 or 1.5 volts. So voltage is 12-1.5 = 10.5 volts.
You could also add the resisters (32K) and calculate the current (12V/32K = 375 mA.) The voltage drop acorss the 4+8+16K (28K) resistance is 28K x 375x10-3 = 10.5
The voltage at location "x" is ______ volts.
6. A 14 bit Analog to Digital Converter covers the range -5 to +5 volts. What's the resolution ? (smallest voltage change that can be detected ) Answer: _. 00060__ volts.
A 14 bit converter has 214 values (0-16383). The total range is 10 volts so each digit corresponds to 10/16384 = 0.6 millivolts
7. A 3.75 x 10-4 M solution in a 2 cm cell absorbs 37% of the light (450 nm) passing through it.
Don't get fooled by extra data. The %T = 63% so A= log10(0.63) =0.20
What is the Absorbance? __0.20___
8. A photomultiplier is a transducer that converts _light (intensity)__ into electrical current.
9. The name LASER comes from the first letter of the five words
Light Amplification by the Stimulated Emission of Radiation
10. The figure shows a demonstration of the photoacoustic effect (1890.) The parts are a burner, a concave reflecting mirror, a rotating chopper (hand operated), a flask of CO gas, and rubber tubing leading to the distant ear of Professor Tyndall.
Describe briefly why the professor hears a clear tone as his graduate student turns the handle.
The obvious answer is that happy graduate students sing (or at least hum) while they work.
The technical answer has these elements
1. 
The graph below represents a gas chromatogram.
Sample: 500 ng of anthracene (peak at 9.5 minutes)
Column: 3 mm ID x 2.0 meters (packed column)
What is H (or HETP) for the column?
II- 2. 
A solution of tap water is analyzed for Ca using Atomic Absorption.
5.000 ml of the water sample is placed in each of five 250 ml volumetric flasks
flasks #1, 2, 3, 4 and 5 also contain 0.00, 2.00, 4.00, 6.00, and 8.00 ml of 0.000150M Ca2+
each flask is then diluted to volume with distilled water and the absorbance is measured
A plot of Absorbance vs. added ml has been provided and the best fit equation is noted.
Compute the concentration of Ca in the water sample.
II-3. A X-ray tube has metal target of an unspecified element and it is operated at 39000 volts.
This atom has shows ionization potentials of
54.230 keV, 19.638 keV, 4.406 keV, 1.101 keV
What is the wavelength of the X-rays produced?
What fraction of the atoms are in the excited state at 1700 Kelvin?
(Answer 2 of the 4 questions below. Each answer is 20 points. Part III has 40 points)
Use a separate blank page for each answer
Include a diagram and label important components
Briefly explain how device works and the role of important components
Be sure to tell what is measured.
1. ICP Spectrometer with Eschelle optics.
2. Scanning Electron Microscope (equipped to detect specific elements in the sample)
3. Fourier Transform Infrared Spectrometer
4. Gas Chromatograph (with capillary column and FID detection.)
Two methods of Voltametric Analysis are Polarography and Cyclic Voltammetry as shown in the two figures below.
Explain what each graph shows--
why does first figure show its general shape (flat, step, level off and rise again) and why all the wiggles?
why does second figure show both positive and negative peaks?
why do both curves show a region of leveling off?
how does each figure change if the concentration of analyte were to change?
(end of exam)