CHM 2290 Spectroscopy XC quiz

E

Beer's law works for___radiation passing through a___solution in which the absorbing species is___in a concentration-dependent equilibrium. A) polychromatic ; dilute ; not participating B) monochromatic; concentrated; not participating C) monochromatic; concentrated; participating D) polychromatic ; dilute ; participating E) monochromatic; dilute; not participating

A

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 1.000 cm, calculate compound X's molar absorptivity. A) 1443 M−1 cm−1 B) 9.926 × 105 M−1 cm−1 C) 5059 M−1 cm−1 D) 735.9 M−1 cm−1 E) 1.44 × 105 M−1 cm−1

1800

A transmittance of 0.3847 is measured for a 0.00023 M solution. Assuming a pathlength of 1.00 cm, calculate the molar absorptivity of the solute. The molar absorptivity of the solute is ________ M-1 cm-1 Note: Enter ONLY the numerical answer in the following format: XXXX Do not enter units. Use the correct number of significant figures.

D

All of the following are TRUE for luminescence, except: A) luminescence is emission of light from an excited state of a molecule. B) luminescence is inherently more sensitive than absorption. C) at low concentration, emission intensity is proportional to analyte concentration. D) at high concentration, emission intensity decreases due to dilution. E) light emitted from a chemical reaction is called chemiluminescence.

C

Fluorescence is: A) a nonradiative transition between states with the same spin, S1 → S0. B) a nonradiative transition between states with different spin, T1 → S0 C) the emission of a photon during a transition between states with the same spin, S1 → S0. D) the emission of a photon during a transition between states with different spin, T1 → S0 E) None of these answers defines fluorescence.

A

For the UV-Vis absorption data below, calculate the concentration for the metals M2+ and N2+ in a river sample if the unknown solution was prepared by diluting 10.00 mL of river water with 10.00 mL of ligand solution and diluted to a final volume of 100.00 mL. Assume the absorbance was measured in a 1.00-cm cell. (see BB #13) A) M2+= 1.54 × 10−5 M and N2+= 8.25 × 10−6M B) N2+= 1.54 × 10−5 M and M2+= 8.24 × 10−6M C) N2+= 1.54 × 10−6 M and M2+= 8.25 × 10−5M D) M2+= 1.5 × 10−5 M and N2+= 8.2 × 10−6M E) N2+= 1.5 × 10−5 M and M2+= 8.2 × 10−6M

E

For the acid-base indicator bromothymol blue, the protonated form, HBB, is yellow and the deprotonated form, BB−, is blue. When both forms are present the indicator has a green color, the shade of green depends on the ratio of HBB:BB−. Use the information below to calculate the concentration of HBB and BB− for a green solution of bromothymol blue. (see BB #12) A) 3.5 × 10−5 M HBB and 9.5 x 10−6 M BB− B) 3.50 × 10−6 M HBB and 9.50 x 10−4 M BB− C) 3.50 × 10−5 M BB− and 9.50 x 10−6M HBB D) 3.51 × 10−5 M HBB and 9.60 x 10−6 M BB− E) 2.33 × 10−5 M HBB and 9.73 x 10−6 M BB−

B

For the plot of absorbance versus wavelength for two compounds, which two wavelengths are most suited for mixture analysis? (see BB #17) A) 515 nm, 515 nm B) 445 nm, 635 nm C) 380 nm, 750 nm D) 515 nm, 635 nm E) 445 nm, 515 nm

D

Phosphorescence is: A) a nonradiative transition between states with the same spin, S1 → S0. B) a nonradiative transition between states with different spin, T1 → S0 C) the emission of a photon during a transition between states with the same spin, S1 → S0. D) the emission of a photon during a transition between states with different spin, T1 → S0 E) None of these answers defines phosphorescence.

B

Reagent blanks are primarily used to: A) align the optics to achieve maximum irradiance B) compensate for any absorbance due to reagents or contamination C) verify reagent purity D) calibrate the spectrophotometer E) test detector sensitivity

C

The basic spectrophotometer is composed of four components. Which component listed below is NOT one of the four? A) sample B) monochromator C) beam splitter D) light source E) light detector

0.0016

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 1.0 cm and a 100% yield. Fe3+ (aq) + SCN- (aq) FeSCN2+ (aq) The iron concentration is _______ M. Enter your numerical answer in the following format X.XXXXXX Do NOT include units. Report your answer to the correct number of sig figs.

B

The part of a spectrophotometer that selects the wavelength to irradiate the sample is called the: A) polychromator. B) monochromator. C) beam splitter. D) light source. E) optode.

A

Which is true for spectroscopy of a mixture? A) The absorbance for a mixture at a particular wavelength is the sum of the absorbances for the components that absorb at the particular wavelength. B) Spectrophotometers can differentiate between mixture components that absorb at the same wavelength. C) Each component in a mixture has the same molar absorptivity at the same wavelength. D) The concentration for each component in the mixture is easily calculated with a set of simultaneous equations if there is significant overlap of individual spectra. E) The concentration for each component in a mixture is easily calculated by least squares for guesses of each component when the individual spectra are well resolved.

B

Which of the following are correct for Beers Law? I. Absorbance increases as concentration increases. II. Absorbance decreases as pathlength increases. III. The molar absorptivity value is chromophore and wavelength specific. IV. The absorption spectrum is a graph showing absorbance changes with wavelength. A) I, II, and III B) I, III, and IV C) II, III, and IV D) I and III E) I, II, III, and IV

C

Which of the following is NOT true for light modeled as a particle? A) A particle of light is the photon. B) The energy of a photon is proportional to the photon's frequency. C) Wavenumber is inversely proportional to photon energy. D) As the wavelength increases, photon energy decreases. E) The product of frequency and Planck's constant equals photon energy.

B

Which of the following is NOT true for light modeled as a wave? A) Frequency is the number of waves that pass a given point in 1 second. B) Wavelength is the distance between the crest and the trough. C) Light travels slower through matter than a vacuum. D) In a vacuum the speed of light is 3.0 × 108 m/s. E) The product of a waves frequency and wavelength is the speed of light.

E

Which of the following is NOT true for the absorption of light? A) Transmittance is the fraction of incident light that is not absorbed. B) Molecules that absorb light are promoted to an excited state. C) A = −log(T) D) Molecules that emit light return to the ground state. E) Irradiance is the energy per second of the light beam.

A

Which of the following is TRUE for emission spectra? A) Excitation wavelength is fixed and the emission wavelengths are scanned. B) Excitation wavelength is scanned and the emission wavelength is fixed. C) Excitation wavelength is fixed and the emission wavelength is fixed. D) Excitation wavelength is scanned and the emission wavelength is scanned. E) None of these answers is true

B

Which of the following is TRUE for excitation spectra? A) Excitation wavelength is fixed and the emission wavelengths are scanned. B) Excitation wavelength is scanned and the emission wavelength is fixed. C) Excitation wavelength is fixed and the emission wavelength is fixed. D) Excitation wavelength is scanned and the emission wavelength is scanned. E) None of these answers is true.

A

Which of the statements below is FALSE for the excited and ground states of molecules? A) Electron spins are opposed in the singlet, S0, excited state. B) Electron spins are parallel in the triplet, T1, excited state. C) Electrons release energy when transitioning from the S1 to the S0 state. D) Electrons gain energy when transitioning from the S0 to the S1 state. E) Molecular geometry in the excited state may be different than the molecular geometry in the ground state.