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Material Type: Exam; Class: Instrumental Analysis; Subject: Chemistry; University: Shepherd University; Term: Unknown 1989;
Typology: Exams
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Fill in the blank. a) If the absorbance of a 0.020 M sample is 0.150 in a 2.0 cm cell, what is the absorbance of a 0.010 M sample of the same type in a 1.0 cm cell? _______________ b) By what factor is the signal-to-noise ratio improved if 1000 spectra are averaged _____. c) At a particular wavelength, a solution that is 0.010 M in A has an absorbance of 0.26 and a solution that is 0.10 M in B has an absorbance of 0.16. What is the absorbance of a solution that is 0.010 M in A and 0.10 M in B? ______________ e) If a spectrophotometer reads 92% T with the blank and 53% T with the sample, what is the absorbance of the sample? ________ f) Two emission lines occur at 772.0 and 772.8 nm. Determine how many lines a grating must have to resolve these emission lines in second order. ________ f. For a particular determination, the slope of the calibration curve was 0.153 L/mg. The standard deviation of the noise level for a blank sample was measured to be 0.022. What is the detection limit in concentration units for this technique? g. Sketch a typical single beam UV spectrometer. Label the various parts and show the light path. Use the back of this page if you need more room h. Sketch the 19 F NMR spectrum for 31 P^19 F 31 H 2. Assume that JPF = 6 Hz. and JHF =2 Hz. i. What are the principal advantages and disadvantages of Fourier Transform Spectroscopy? j. What would allow you to distinguish the 1 H NMR spectrum of CH 3 CH 2 OH from CH 3 CH 2 OCH 3? k. What are the advantages of a double beam instrument as compared to a single beam instrument? l. Briefly discuss Beer's Law. What is it and why does it sometimes fail? m. How many spectra must be averaged to increase the signal to noise ratio by a factor of 5.
n. What is the maximum resolution in nm for a 2048 element multichannel detector if the wavelength range covered is 200 nm to 400 nm. o. If a 0.010 M sample has a transmittance of 90% what is the concentration of a solution of the same substance that has a %transmittance of 80%. Assume that Beer’s Law applies. p. Convert 500 nm to the equivalent wavenumber (cm-1) ________ q. What is the energy difference between two levels if the number of molecules in the upper state is one fourth as many as in the lower state. Assume the temperature is 500 K _______. Select the technique or techniques from the list below that would be appropriate for each of the following analyses. Indicate if any special sample preparation is needed. (transmission IR, reflectance IR, reflectance near-IR, Raman, UV-vis, fluorescence, NMR) a) determine if a water solution has a small benzene impurity b) determine the CO 2 content of an air sample. c) distinguish between 1,2- and 1,3- dichlorobenzene d) determine Fe3+^ concentration in a water sample
Which one of the following statements is FALSE At the detection limit, the signal-to-noise ratio is about 3 The slope of a calibration curve is the sensitivity One of the reasons to use an internal standard is to eliminate matrix effects The accuracy of a Beer’s law plot decreases at both high and low absorbance. Which one of the following is NOT an advantage of Raman spectroscopy as compared to IR spectroscopy. preparing a sample is usually easier in Raman the Raman detection limit is lower biological samples are easier to study in Raman Raman is better for detecting symmetric vibrations A typical source for an IR experiment is a tungsten lamp a globar an RF coil a D 2 lamp
Which one of the following is NOT a type of multichannel detector? photodiode array charge coupled device photographic film photomultiplier In the figure shown above, which arrow corresponds to Raman emission? A C F H In the figure shown above, which arrow corresponds to UV or visible absorption? A B G H In the figure shown above, which arrow corresponds to IR absorption? A B F H In the figure shown above, which arrow corresponds to fluorescence? A C F H Which one of the following techniques is LEAST used as a quantitative method? fluorescence UV-Vis near-IR NMR
Which substance could not be detected by infrared spectroscopy NaCl water benzene ethanol Which one of the following is not a detector for visible light vacuum phototube photomultiplier MCT detector silicon photodiode
The signal observed in FT-NMR is called the spin-spin relaxation free induction decay spin-lattice relaxation Fourier transform Molar absorptivity varies with cell path length wavelength concentration detector Which pair of the following pair of spectroscopic techniques would be most useful for the determination of the structure of an unknown organic compound? NMR and IR NMR and UV-Vis IR and UV-Vis UV-Vis and fluorescence The term refers to the raw signal observed in FT-NMR is free induction decay spin-spin splitting spin-lattice relaxation DEPT In FT-NMR the sample is irradiated with pulses of radiation because? a short pulse can simultaneously excite all nuclei of a given type a pulse of high power radiation is more efficient than continuous radiation at the power levels used continuous radiation would cause saturation the use of pulses permits greater precision in the frequency that is applied Which group of instruments are most likely to be able to use the same type of detector? IR, Raman and UV-Vis Raman, fluorescence and UV-Vis IR, fluorescence and AA IR, fluorescence and UV-Vis Which one of the following pairs are common IR detectors? PMT and MCT MCT and DTGS DTGS and CCD PMT and CCD Which part of the electromagnetic spectrum is used in NMR spectroscopy? visible microwave UV radio wave Which one of the following parts of the IR spectrum tends to have the most information on characteristic group frequencies 200 - 800 nm 400 - 2000 cm^ 4000 - 14000 cm^ 20 - 400 cm Which of the following terms describes a radiationless processe for an electronic transition from a singlet to triplet state?