Deck 19: Fundamentals of Spectrophotometry

Question 1

Absorbance measured during a spectrophotometric titration must be corrected due to

A)absorbing wavelength changes over time.
B)dilution of the analyte by titrant.
C)contaminants in the titrant.
D)instrument drift.
E)decomposition of analyte.

Answer 1

dilution of the analyte by titrant.

Question 2

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

Answer 2

monochromatic;dilute;not participating

Question 3

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.

Answer 3

compensate for any absorbance due to reagents or contamination.

Question 4

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.

Question 5

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.

Question 6

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.

Question 7

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.

Question 8

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.

Question 9

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

Question 10

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)1,III,and IV
C)II,III,and IV
D)I and III
E)I,II,III,and IV

Question 11

A 10.00 mL sample is titrated with 0.50 mL of titrant.The observed absorbance is 0.3219.Calculate the corrected absorbance.

A)0)3066
B)0)3058
C)0)2912
D)0)3380
E)0)3558

Question 12

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.

Question 13

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.

Question 14

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)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

Question 15

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.

Question 16

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.

Question 17

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.

Question 18

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.

Question 19

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)

Question 20

Calculate the energy of a photon with a frequency of 573.8 nm.

A)3)46 x 10⁻³⁷ J
B)3)80 x 10⁻⁴⁰ J
C)3)46 x 10⁻²⁸ J
D)3)80 x 10⁻³¹
E)3)46 x 10⁻¹⁹ J

Question 21

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)

Question 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?

Question 23

After 10.00 mL titrant has been added to 50.00 mL analyte the absorbance was measured to be 0.4639.Calculate corrected absorbance.