UMD CHEM 425 - sample_2_o14 (6 pages)

INSTRUMENTAL METHODS OF ANALYSIS MID TERM EXAM II XXX 125 Points CALCULATIONS 60 Points 1 20 points A mixture of chelating reagent B with Ni II gives rise to the formation of a highly colored NiB22 complex solutions of which obey Beer s law over a wide range Provided the analytical concentration of the chelating reagent exceeds that of Ni II by a factor of five or more the cation exists within the limits of observation entirely in the form of the complex Use the accompanying data to evaluate the formation constant Kf for the reaction Ni2 2 B NiB22 Concentrations M Ni2 B Absorbance A 395 nm 1 cm cell 2 5x10 4 2 20x10 1 0 765 2 5x10 4 1 00x10 3 0 360 2 25 points When measured with a 1 00 cm cell a 7 50 x 10 5 M solution of species A exhibited absorbances of 0 155 and 0 755 at 475 and 700 nm respectively A 4 25 x 10 5 M solution of species B gave absorbances of 0 702 and 0 091 under the same circumstances Calculate the concentration of A and B in a solution that yielded the following absorbance data in a 2 5 cm cell 0 439 at 475 nm and 1 025 at 700 nm 3 15 points If the fluorescence quantum yield and lifetime of compound X were 0 5 and 200 nanoseconds respectively under anaerobic conditions what would the quantum yield and lifetime be under aerated conditions assuming that O2 quenches the excited singlet state with a rate constant of 1x1010 M 1s 1 and the solution concentration of O2 is 1 5 mM When the solution concentration of O2 is 250 M INSTRUMENTS 25 points 2 A 5 points Provide a block diagram of a photodiode array UV Vis absorption spectrophotometer clearly labeling each of the components B 5 points Provide a diagram of a monochromator clearly labeling each of the components C 5 points Provide a block diagram of a fluorescence spectrometer clearly labeling each of the components D 5 points Provide a block diagram of a mass spectrometer clearly labeling each of the components E 5 points Provide a block diagram of a single beam scanning UV Vis scanning spectrophotometer clearly labeling each of the components SHORT ANSWERS 40 points Mass Spectrometry 18 points 3A 2 points Name two gas phase ionization sources B 2 points What ionization source is used in the mass spectrometer in the 425 laboratory C 2 points In chemical ionization which reaction proton transfer hydride abstraction or charge exchange produces the M 1 ion Which reaction produces the M 1 ion D 2 points Name two ionization sources that are used most commonly for the analysis of high molecular weight polar compounds such as proteins E 2 points Name four mass analyzers F 2 points What mass analyzer is employed in the GC MS in the 425 laboratory G 2 points For what reason is this mass analyzer employed in the GC MS in the 425 laboratory H 4 points If a mass analyzer had a resolution of 10 000 at an m z 500 would you be able to resolve masses at 500 00 and 500 05 You must show your calculation to obtain credit for your answer Ultraviolet Visible Absorption Spectrophotometry 12 points 4A 2 points Provide an equation that relates absorbance to concentration What is this equation called B 4 points Name two instrumental factors that can produce deviations from the relationship provided in Question A C 2 points Name two types of electronic transitions that have very high molar absorptivities D 2 points What two light sources are commonly employed in UV Vis spectrophotometry to provide light in the ultraviolet and visible wavelength ranges respectively E 2 point What detector is employed in the spectrophotometer in the 425 lab Fluorescence Spectroscopy 10 points 5A 2 points Under what condition is fluorescence intensity proportional to concentration B 3 points What is the ratio mode in spectrofluorometry and why is it employed C 2 points Define fluorescence quantum yield in either words or an equation D 3 points Name three possible relaxation pathways for a molecule in its first excited singlet state