Fundamentals of Spectrophotometry Answer Key

Chapter 18 – Fundamentals of Spectrophotometry Multiple-Choice 18-1 Properties of Light 1. Which of the following is NOT true for light modeled as a wave? A) B) C) D) E) Frequency is the number of waves that pass a given point in 1 second. Wavelength is the distance between the crest and the trough. Light travels slower through matter than a vacuum. In a vacuum the speed of light is 3.0 × 108 m/s. The product of a waves frequency and wavelength is the speed of light. Answer: B Easy 2. Which of the following is NOT true for light modeled as a particle? A) B) C) D) E) A particle of light is the photon. The energy of a photon is proportional to the photon’s frequency. Wavenumber is inversely proportional to photon energy. As the wavelength increases, photon energy decreases. The product of frequency and Planck’s constant equals photon energy. Answer: C Easy 3. Calculate the energy of a photon with a frequency of 573.8 nm. A) B) C) D) E) 3.46 x 10−37 J 3.80 x 10−40 J 3.46 x 10−28 J 3.80 x 10−31 3.46 x 10−19 J Answer: E Intermediate 1 Chapter 18 – Fundamentals of Spectrophotometry 18-2 Absorption of Light 4. Which of the following is NOT true for the absorption of light? A) B) C) D) E) Transmittance is the fraction of incident light that is not absorbed. Molecules that absorb light are promoted to an excited state. A = −log(T) Molecules that emit light return to the ground state. Irradiance is the energy per second of the light beam. 2 Answer: E Intermediate 5. Which of the following are correct for Beers Law? I II III IV Absorbance increases as concentration increases. Absorbance decreases as pathlength increases. The molar absorptivity value is chromophore and wavelength specific. The absorption spectrum is a graph showing absorbance changes with wavelength. A) B) C) D) E) I, II, and III 1, III, and IV II, III, and IV I and III I, II, III, and IV Answer: B Intermediate 6. A 3.7 × 10−4 M solution of compound X has an absorbance of 0.5342 when measured at 740 nm. Assuming a pathlength of 1 cm, calculate compound X’s molar absorptivity. A) B) C) D) E) 1443 M−1 cm−1 9.926 × 105 M−1 cm−1 5059 M−1 cm−1 735.9 M−1 cm−1 1.44 × 105 M−1 cm−1 Answer: A Intermediate Chapter 18 – Fundamentals of Spectrophotometry radiation passing through a 7. Beer’s law works for in a concentration-dependent equilibrium. absorbing species is A) B) C) D) E) 3 solution in which the polychromatic ; dilute ; not participating monochromatic; concentrated; not participating monochromatic; concentrated; participating polychromatic ; dilute ; participating monochromatic; dilute; not participating Answer: E Intermediate 18-3 Measuring Absorbance 8. The basic spectrophotometer is composed of four components. Which component listed below is NOT one of the four? A) B) C) D) E) sample monochromator beam splitter light source light detector Answer: C Intermediate 9. the: The part of a spectrophotometer that selects the wavelength to irradiate the sample is called A) B) C) D) E) polychromator. monochromator. beam splitter. light source. optode. Answer: B Easy Chapter 18 – Fundamentals of Spectrophotometry 18-4 Beer’s Law in Chemical Analysis 10. Reagent blanks are primarily used to: A) B) C) D) E) align the optics to achieve maximum irradiance. compensate for any absorbance due to reagents or contamination. verify reagent purity. calibrate the spectrophotometer. test detector sensitivity. Answer: B Intermediate 18-6 What Happens When a Molecule Absorbs Light? 18-7 Luminescence 16. Which of the following is TRUE for emission spectra? A) B) C) D) E) Excitation wavelength is fixed and the emission wavelengths are scanned. Excitation wavelength is scanned and the emission wavelength is fixed. Excitation wavelength is fixed and the emission wavelength is fixed. Excitation wavelength is scanned and the emission wavelength is scanned. None of these answers is true. Answer: A Intermediate 16. Which of the following is TRUE for excitation spectra? A) B) C) D) E) Excitation wavelength is fixed and the emission wavelengths are scanned. Excitation wavelength is scanned and the emission wavelength is fixed. Excitation wavelength is fixed and the emission wavelength is fixed. Excitation wavelength is scanned and the emission wavelength is scanned. None of these answers is true. Answer: B Intermediate 17. All of the following are TRUE for luminescence, except: A) B) C) D) luminescence is emission of light from an excited state of a molecule. luminescence is inherently more sensitive than absorption. at low concentration, emission intensity is proportional to analyte concentration. at high concentration, emission intensity decreases due to dilution. 4 Chapter 18 – Fundamentals of Spectrophotometry E) 5 light emitted from a chemical reaction is called chemiluminescence. Answer: D Intermediate Calculated 18-1 Properties of Light 18. A red laser emits a pulse of light every 10 s. If the laser operates at a wavelength of 695 nm and 50 W, calculate the number of photons emitted during a pulse. (1 W = 1 J/s) Answer: 1.75 × 1015 photons Hard Calculate the energy of one photon (2.86 × 10-19 J/photon), then the total energy of one pulse (0.0005 J), then total energy divided by energy by photon (1.75 × 1015.) 18-2 Absorption of Light 19. A transmittance of 0.3847 is measured for a 0.00023 M solution. Assuming a pathlength of 1 cm, calculate the molar absorptivity of the solute. Answer: 1803 M−1 cm−1 Intermediate Calculate absorbance from transmittance (0.4149), then molar absorptivity. 18-4 Beer’s Law in Chemical Analysis 20. The iron content of a well water sample must be quantified. A 10.00 mL aliquot of well water is transferred to a 100.0-mL volumetric flask and 20 mL of 0.10 M nitric acid is added to oxidize iron(II) to iron(III). Next 30 mL of a 0.1 M sodium thiocyanate solution is added to form the deep red complex iron(III) thiocyanate. The solution is diluted to volume with distilled deionized water. A reagent blank is similarly prepared. If the absorbance of the iron thiocyanate solution is 0.5483 and the reagent blank absorbance is 0.0282, calculate the iron concentration for the water sample. The molar absorptivity for iron(III) thiocyanate is 3200 M−1 cm−1. Assume a pathlength of 1 cm and a 100% yield. Fe3+ (aq) + SCN- (aq) ↔ FeSCN2+ (aq) Answer: 0.001625 M Hard Calculate corrected absorbance (0.5201), then the concentration iron (0.0001625 M) and then take into account the dilution factor (1:10.) 18-5 Spectrophotometric Titrations 21. After 10.00 mL titrant has been added to 50.00 mL analyte the absorbance was measured to be 0.4639. Calculate corrected absorbance. Chapter 18 – Fundamentals of Spectrophotometry 6 Answer: 0.5567 Easy 18-6 What Happens When a Molecule Absorbs Light? 22. Quinine sulfate in 0.5 M sulfuric acid is excited at 310 nm and fluoresces at 450 nm. How many Joules were dissipated through nonradiative transitions on a per molecule basis? Answer: 1.99 × 10−19 J dissipated Hard Either calculate E310 and E450, subtracting E450 from E310 to get energy dissipated, Edisp. Or 1/310 −1/× = 1/450, solving for ×, 996.4 which is the wavelength for the energy difference, 1.99 × 10−19 J. Chapter 20 – Spectrophotometers 1 Multiple-Choice 1. Which of the following statements regarding double beam spectrophotometers is NOT true? A) B) C) D) E) The reference beam allows for automatic correction for source intensity fluctuations. Comparison of the power emerging from the sample and reference beams determines absorbance and transmittance. The reference beam allows for automatic correction for detector drift. The reference beam is created using a stationary mirror. The reference beam is created using a rotating mirror. Answer: D Intermediate 20-1 Lamps and Lasers: Sources of Light 2. Spectrophotometers employ various sources of light. Which of the following is NOT true for spectrophotometer light sources? A) B) C) D) E) Tungsten lamps emit radiation over the visible spectrum and parts of the ultraviolet and infrared spectrums. Dueterium arc lamps use controlled electric discharges to cause D2 to dissociate and emit visible radiation. Light emitting diodes emit narrow bands of visible and near-infrared radiation. Lasers emit radiation of a single wavelength. A silicon carbide globar emits infrared radiation when heated to near 1500 K. Answer: B Intermediate 3. Which property of laser light is NOT correctly defined/described? A) B) C) D) E) monochromatic: emits one wavelength of light extremely bright: high power at one wavelength collimated: parallel rays of light polarized: electric field oscillates between two perpendicular planes coherent: all waves in phase Answer: D Easy Chapter 20 – Spectrophotometers 20-2 Monochromators 4. Which of the following monochromator terms is NOT correctly defined? A) B) C) D) E) grating: optical element with closely spaced lines diffraction: bending of light by a grating refraction: scattering of light by a lens or prism polychromatic: multiple wavelengths monochromatic: one wavelength 2 Answer: C Intermediate 5. Stray light is a problem for spectrophotometers that are not tightly sealed. What is the impact of stray light on the recorded absorbance? A) B) C) D) E) The apparent absorbance is greater than true absorbance and increases as the amount of stray light increases. The apparent absorbance is greater than true absorbance and decreases as the amount of stray light increases. The apparent absorbance is less than the true absorbance and increases as the amount of stray light decreases. The apparent absorbance is less than the true absorbance and decreases as the amount of stray light decreases. Impossible to determine effect of stray light on measured absorbance. Answer: C Intermediate Chapter 20 – Spectrophotometers 3 disperses light into its component wavelengths and selects a narrow 6. A band of wavelengths to pass on to the sample or detector. A) B) C) D) E) polychromator photodiode array monochromator charge couple device photomultiplier tube Answer: C Easy 20-3 Detectors 7. Which statement(s) is (are) true for photomultiplier tubes? I Photomultiplier tubes are very sensitive detectors, amplifying the photoelectron current by approximately 1 million. II Emitted photoelectrons are accelerated toward positively charged electrodes called dynodes. III Electrons strike the dynode with a kinetic energy less than the original emitted kinetic energy. IV For each electron striking a dynode, more than one electron is knocked free. The additional electrons are accelerated toward the next dynode in the detector where amplification is repeated. A) B) C) D) E) II I and IV II, III, and IV I, II, and IV II and III Answer: D Intermediate 8. Which is NOT an appropriate detector for a spectrophotometer? A) B) C) D) E) photomultiplier tube thermal conductivity photodiode array charged coupled device phototube Answer: B Intermediate 9. Which is NOT true for photodiode arrays? Chapter 20 – Spectrophotometers A) B) C) D) E) 4 Photodiode arrays respond to more than one wavelength. Photodiode arrays require a polychromator to detect more than one wavelength. Photodiode arrays allow for rapid acquisition of an entire light spectrum. Photodiode arrays have higher signal-to-noise ratios. Spectrophotometers using photodiode arrays are dispersive spectrometers. Answer: E Intermediate 10. current when struck by analyte concentration. A) B) C) D) E) response is a function of incident light and produce an electrical . For a given radiant power, current is Optical ; photons; constant for varying Detector; photons; proportional to Analyte; analyte particles; constant for varying Detector; analyte particles; inversely proportional to Optical ; analyte particles; inversely proportional to Answer: B Easy