Chemistry 360 – April 2002 – T. Smith-Palmer Page 2
April 2002 Chemistry 360 T. Smith-Palmer
Exam
Please read through the exam for 5 minutes before starting
1. A group of organic chemists have come to a seminar at the Charles Keating Millenium Centre on Instrumental Separation Methods of use to organic chemists. The lectures will be spread over 3 days. You are requested to help prepare the schedule and course outlines, as follows:
[22] Prepare a list of four instruments that will be discussed. Each should have the ability to analyze organic compounds. For each instrument, provide a brief outline of
[2] (a) How it achieves the separation/differentiation.
[1] (b) What restrictions there are on the type of compounds that can be analyzed.
[1] (c) Give an example of an analysis which could be carried out on the instrument in question ( suggested example should contain at least two species to analyze).
[1.5] (d) Draw a cartoon diagram of the instrument. Label all parts as specifically as possible.
Note: the marks shown by (a), (b), (c) and (d) are for each instrument. [22] is the total marks for this section
[16] 2. I am now going to ask you to prepare outlines – in note form, for 2 of the following lectures. Use headings and sub-headings and brief notes
Choose two from the following 3 topics:
(a) The Ultimate detector – design, function and interpretation
(b) Columns for Gas Chromatography including ways to characterize them.
(c) Theory of Chromatographic Separations including band-broadening.
[26] 3. At ChemTech research labs, the inorganic chemists that are left behind from the conference are deciding what instrumentation they could have in the lab to do analyses of cations and anions. They have enough money to buy 3 new instruments that can analyse mixtures of ions. Suggest what 3 instruments the lab should buy.
For each,
[2.5] (a) Describe the means by which the species are separated or distinguished.
[2.5] (b) Describe the method of detection.
[1] (c) Can the instrument be used for cation or anion analysis, or both? If both – can they be analyzed together or must it be in different analyses?
[1] (d) Name 2 inorganic species that could be analyzed by the instrument.
[1] (e) What limitations are there as to the species that can be analyzed.
Note: the marks shown by (a), (b), (c), (d) and (e) are for each instrument. [26] is the total marks for this section
Do three of the following problems. [6 points each]
4. Caffeine and ASA were separated on a 10 cm chromatography column. The chromatogram had ideal Gaussian bands with tr’s of 6.0 minutes and 7.0 minutes for caffeine and ASA respectively. They have base widths of 0.5 min and 0.9 min respectively.
(a) How many theoretical plates does the column have in this separation?
(b) What is the resolution? Are caffeine and ASA completely resolved?
(c) What length of column would be required to double the resolution? Is this feasible – explain your answer.
5. An unknown sample gave a polarographic signal (DPP) of 8.50 μA for Cd2+. When 0.200 mL of 0.0250 M Cd2+ standard was added to 10 mL of the unknown, the signal increased to 12.0μA. Find the concentration of Cd2+in the unknown.
There is another signal in the polarogram of the unknown. How would we decide what the species causing it might be?
6. A radionuclide has an initial activity of 2.0 ´ 106 dis/min, and after 4.0 days its activity is 9.0 ´ 105 dis/min. Calculate the activity of the sample after 40 days.
7. The infrared spectrum of a flavour compound isolated from apples showed that it was an aldehyde. The mass spectrum had the following major peaks:
m/z relative abundance
100 5
82 20
72 33
57 62
56 85
44 100
The 101 peak had a height that was 6.9% of the m/z =100 peak.
What is the structure of the compound.
Explain how each peak arises, and how the structure was arrived at.
8. Co2+ + 2e- ® Co E° = -0.28 V
O2 + 4e - + 4H+ ® 2H2O Eo = 1.23 V
Co is being deposited at a Pt electrode from a solution in a beaker buffered at pH 4, with a [Co2+] of 0.2M. A voltage is applied and the current is held at 20 mA. The resistance of the cell is 25 ohms and the area of each Pt electrode is 20 cm2. The activation overpotentials for H2 and O2 formation at Pt are 0.024 V and 0.721 V respectively at a current density of 10 A/m2. Use aO2 =1 and aH2 = 1.
What is the cell potential (not just the cathodic potential) required for initial deposition of Co ?
What is the cell potential when H2 formation will occur?
What is the problem?
What could be changed so that cobalt can be successfully deposited?
[18] 9. Do 9 of the following. [2 point each]
(a) Explain the difference between charging current and faradaic current. In polarography, how can we ensure we only measure the desired component of current?
(b) Why does the reaction Cu(I) ® Cu(0) have different half wave potentials with Hg and Pt electrodes.
(c) Sketch the V-t curve for differential pulse polarography. Indicate drop formation and when current is measured.
(d) Briefly describe how a thermionic emission detector works.
(e) Briefly describe how an electron capture detector works.
(f) Briefly describe how an evaporative light scattering detector works.
(g) Solvent passes through a column in 3.0 min but solute requires 9 min. Calculate the capacity factor.
(h) State 5 ways to achieve differential hold-up in chromatography. Elaborate on one of these.
(i) Give an example of a gradient for reverse phase polorography. Sketch a chromatogram which could be improved by using a gradient, and sketch another chromatogram for which the use of a gradient would be inappropriate.
(j) What are the advantages of using chemical ionization rather than electron impact ionization? Briefly distinguish between the two.
(k) The mass spectrum of 1-bromobutane has a pair of nearly equal-intensity peaks at 136 and 138 mass units and another pair at 107 and 109. The peaks at 57 and 41 have no similar equal-intensity partners. Explain why this is so.
(l) Describe an electrospray HPLC-MS interface.
(m) What is the purpose of supporting electrolyte in polarography. Give an example of a supporting electrolyte.
(n) How are neutral species separated by CE?
(o) What is an alpha particle?
Write an equation showing what happens to 238U when alpha decay occurs.
What is a beta particle?
Write an equation showing what happens to 90Sr when beta decay occurs.
Some atomic numbers you might need: Sr –38, Y-39, Rb-37
Ac-89, Th-90, Pa-91, U-92, Np-93, Pu-94
(p) Describe anodic stripping voltammetry – describe the set-up necessary and what is actually happening.