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.

Accepted Answer

As titrant is added to the analyte solution, the concentration of the analyte is diluted, leading to a decrease in absorbance. This must be corrected for accurate results.

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

Accepted Answer

Beer's Law, also known as Beer-Lambert Law, applies when monochromatic (single wavelength) radiation passes through a dilute solution. The absorbing species in the solution must be in a concentration-dependent equilibrium, but it does not participate in any reaction during the absorption process, hence "not participating" is correct.

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.

Accepted Answer

Reagent blanks are used to compensate for any absorbance due to reagents or contamination. They are prepared using the same reagents as the actual sample, but without the analyte. The blank solution is used to set the baseline of the instrument to zero. This helps in correcting any interference due to reagents, as the blank solution acts as a reference point.

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.

Accepted Answer

At high concentration, emission intensity decreases due to self-quenching, not dilution.

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.

Accepted Answer

Irradiance is defined as the power of electromagnetic radiation per unit area (surface power density), which is energy per second per unit area, not just the energy per second of the light beam. The definition provided in E is more accurately describing power or intensity rather than irradiance, which necessitates specifying the area over which the power is distributed.

Question 6

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

Accepted Answer

Fluorescence is the emission of a photon during a transition between states with the same spin. Choice A and B refer to nonradiative transitions, which do not involve the emission of a photon, and Choice D refers to transitions between states with different spin. Therefore, Choice C is the best choice for defining fluorescence.

Question 7

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

Accepted Answer

Phosphorescence involves the emission of a photon during a transition between states with different spin, typically from a triplet state to a singlet state.

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.

Accepted Answer

Wavelength is actually defined as the distance between successive crests (or troughs) of a wave, not the distance between a crest and a trough.

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

Accepted Answer

A beam splitter is not a basic component of a spectrophotometer. The four main components are the light source, monochromator, sample holder, and 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

Accepted Answer

I and IV are correct because Beer's Law states that absorbance is directly proportional to the concentration of the absorbing species and the path length through which the light passes. III is correct because the molar absorptivity (or extinction coefficient) is a measure of how strongly a chemical species absorbs light at a given wavelength, which is specific to each chromophore. II is incorrect because according to Beer's Law, absorbance increases with path length, not decreases.

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

Accepted Answer

The answer of A 10.00 mL sample is titrated with...

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.

Accepted Answer

A monochromator is a device that selects a narrow band of wavelengths of light from a broader spectrum and directs it towards the sample. It is a critical component of a spectrophotometer.

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.

Accepted Answer

C) is not true because wavenumber is directly proportional to photon energy, not inversely. The energy of a photon is given by E = hf, where h is Planck's constant and f is the frequency. Since frequency is inversely proportional to wavelength (λ), and wavenumber is defined as 1/λ, it implies that as wavenumber increases, the frequency increases, and thus the energy of the photon increases as well.

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

Accepted Answer

The molar absorptivity (ε) can be calculated using Beer-Lambert Law: A = εlc, where A is absorbance, ε is molar absorptivity, l is path length in cm, and c is concentration in M. Given A = 0.5342, l = 1 cm, and c = 3.7 × 10^-3 M, solving for ε gives ε = A / (lc) = 0.5342 / (1 cm × 3.7 × 10^-3 M) = 144.378 M^-1 cm^-1, which rounds to 1443 M^-1 cm^-1, matching choice A.

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.

Accepted Answer

In the singlet excited state (S1), electron spins are opposed, not in the singlet ground state (S0).

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.

Accepted Answer

In excitation spectra, the excitation wavelength is scanned while the emission wavelength is fixed. This allows the identification of the optimal excitation wavelength for a particular fluorophore.

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.

Accepted Answer

Emission spectra involve fixing the excitation wavelength and scanning across different emission wavelengths to observe the spectrum of light emitted by the substance.

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.

Accepted Answer

The answer of A transmittance of 0.3847 is measured for...

Question 19

A red laser emits a pulse of light every 10 $\mu$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)

Accepted Answer

The answer of A red laser emits a pulse of...

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

Accepted Answer

The energy of a photon is calculated using the formula E = hν, where h is Planck's constant (6.626 x 10^-34 J·s) and ν is the frequency. First, convert the wavelength (573.8 nm) to meters and then use the speed of light (c = 3 x 10^8 m/s) to find the frequency. Finally, calculate the energy using the formula.

Quiz 19: Fundamentals of Spectrophotometry

Absorbance measured during a spectrophotometric titration must be corrected due to

Beer's law works for radiation passing through a solution in which the absorbing species is in a concentration-dependent equilibrium.

Reagent blanks are primarily used to:

All of the following are TRUE for luminescence,except:

Which of the following is NOT true for the absorption of light?

Fluorescence is:

Phosphorescence is:

Which of the following is NOT true for light modeled as a wave?

The basic spectrophotometer is composed of four components.Which component listed below is NOT one of the four?

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

The part of a spectrophotometer that selects the wavelength to irradiate the sample is called the:

Which of the following is NOT true for light modeled as a particle?

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.

Which of the statements below is FALSE for the excited and ground states of molecules?

Which of the following is TRUE for excitation spectra?

Which of the following is TRUE for emission spectra?

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.

A red laser emits a pulse of light every 10 $\mu$ 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)

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

Quiz 19: Fundamentals of Spectrophotometry

Absorbance measured during a spectrophotometric titration must be corrected due to

Beer's law works for____________________ radiation passing through a____________________ solution in which the absorbing species is____________________ in a concentration-dependent equilibrium.

Reagent blanks are primarily used to:

All of the following are TRUE for luminescence,except:

Which of the following is NOT true for the absorption of light?

Fluorescence is:

Phosphorescence is:

Which of the following is NOT true for light modeled as a wave?

The basic spectrophotometer is composed of four components.Which component listed below is NOT one of the four?

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

The part of a spectrophotometer that selects the wavelength to irradiate the sample is called the:

Which of the following is NOT true for light modeled as a particle?

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.

Which of the statements below is FALSE for the excited and ground states of molecules?

Which of the following is TRUE for excitation spectra?

Which of the following is TRUE for emission spectra?

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.

A red laser emits a pulse of light every 10 μ\muμ 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)

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

Beer's law works for radiation passing through a solution in which the absorbing species is in a concentration-dependent equilibrium.

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 red laser emits a pulse of light every 10 $\mu$ 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)