Spectroscopy Absorbance Practice Exercises
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Questions and Answers

What is the unit of frequency?

  • nm (nanometer)
  • Hertz (Hz) (correct)
  • second (s)
  • meter (m)

    • What is electromagnetic radiation (EMR)?

  • A type of chemical reaction
  • A form of energy that consists of oscillating electric and magnetic fields (correct)
  • A type of chemical substance
  • A measure of the absorption or emission of energy

    • What is amplitude?

  • The number of waves that pass a fixed point per second
  • The maximum vertical displacement of the wave from normal position (correct)
  • The reciprocal of the wave length
  • The distance between two identical peaks or troughs in a wave

    • What is wavelength?

    <p>A measure of distance between two identical peaks or troughs in a wave</p> Signup and view all the answers

    What is wave number?

    <p>The reciprocal of the wave length</p> Signup and view all the answers

    What is the term for the measurement of the absorption or emission of electromagnetic radiation by a chemical substance?

    <p>Qualitative analysis</p> Signup and view all the answers

    What is the frequency of the radiation produced by a laser used in eye surgery to fuse detached retinas, given its wavelength is 640.0 nm?

    <p>4.688×10^14 s^-1</p> Signup and view all the answers

    What is the energy of a photon of radiation from a laser emitting light with a frequency of 4.69 x 10^14 s^-1?

    <p>3.374 x 10^-19 J</p> Signup and view all the answers

    What is the wavelength of electromagnetic radiation broadcasted by an FM radio station at a frequency of 103.4 MHz?

    <p>2.901 m</p> Signup and view all the answers

    What is the total energy of a pulse of radiation containing 5.0 x 10^17 photons, if the energy of one photon is 3.374 x 10^-19 J?

    <p>1.687 x 10^1 J</p> Signup and view all the answers

    What is the speed of light used in the calculation of frequency and wavelength?

    <p>3.00 x 10^8 m/s</p> Signup and view all the answers

    What is the Planck's constant used in the calculation of energy of a photon?

    <p>6.626 x 10^-34 J.s</p> Signup and view all the answers

    What is the relationship between percent transmittance and transmittance?

    <p>Percent transmittance is 100 times the transmittance</p> Signup and view all the answers

    If the absorbance of a sample is 0.5, what is its percent transmittance?

    <p>63.2%</p> Signup and view all the answers

    What is the unit of molar absorptivity?

    <p>M^-1 cm^-1</p> Signup and view all the answers

    What type of transition is allowed in CH4?

    <p>σ → σ*</p> Signup and view all the answers

    What is the wavelength of maximum absorption for CH4?

    <p>125 nm</p> Signup and view all the answers

    What is the formula for absorbance?

    <p>A = -log T</p> Signup and view all the answers

    What is the purpose of UV-Vis molecular absorption spectrometry?

    <p>To perform qualitative and quantitative analyses</p> Signup and view all the answers

    What is the relationship between absorbance and molar absorptivity?

    <p>A = εlc</p> Signup and view all the answers

    What is the energy of γ -Ray radiation?

    <p>3.11× 10-19 J</p> Signup and view all the answers

    What happens to the wavelength of EMR as the frequency increases?

    <p>It decreases</p> Signup and view all the answers

    What type of radiation affects the atomic nuclei?

    <p>γ -Ray</p> Signup and view all the answers

    What is the color of an object that absorbs all wavelengths in the visible region of the spectrum?

    <p>Black</p> Signup and view all the answers

    What is the type of radiation that causes molecular vibration?

    <p>IR</p> Signup and view all the answers

    What is the relationship between the energy and frequency of EMR?

    <p>They are directly proportional</p> Signup and view all the answers

    What is the effect of X-Ray radiation on matter?

    <p>It affects inner electronic transitions</p> Signup and view all the answers

    What is the type of radiation that causes molecular rotation?

    <p>Microwave</p> Signup and view all the answers

    What type of transitions occur in saturated compounds containing atoms with unpaired electrons?

    <p>n → σ* transitions</p> Signup and view all the answers

    What is the wavelength range for absorption bands of organic compounds due to electronic transitions?

    <p>200-700 nm</p> Signup and view all the answers

    What type of organic functional groups absorb in the ultraviolet or visible region?

    <p>Chromophores</p> Signup and view all the answers

    What is the type of transition that occurs in metal-ligand complexes?

    <p>Charge-transfer transition</p> Signup and view all the answers

    What is the reason for the red color of the iron(III)/thiocyanate complex?

    <p>Charge-transfer absorption</p> Signup and view all the answers

    What type of transition occurs in d-d transitions?

    <p>Transitions between d orbitals of metal</p> Signup and view all the answers

    Why is the determination of an analyte's concentration based on its absorption of ultraviolet or visible radiation a frequently encountered quantitative analytical method?

    <p>Many organic and inorganic compounds have strong absorption bands in the UV/Vis region</p> Signup and view all the answers

    What can be done if an analyte does not absorb UV/Vis radiation or its absorbance is too weak?

    <p>React it with another species that is strongly absorbing</p> Signup and view all the answers

    Study Notes

    Absorbance and Transmittance

    • Absorbance (A) is related to transmittance (T) by the equation A = -log T
    • A percent transmittance of 50% is equivalent to a transmittance of 0.500, which corresponds to an absorbance of 0.301
    • If the absorbance is 1.27, the transmittance is 0.054, which corresponds to a percent transmittance of 5.4%

    Molar Absorptivity

    • Molar absorptivity (ε) is a measure of the absorption of a substance at a given wavelength
    • The equation A = εlc can be used to calculate the molar absorptivity of an analyte, where A is the absorbance, ε is the molar absorptivity, l is the pathlength of the sample cell, and c is the concentration of the analyte
    • A 5.00 × 10^(-4) M solution of an analyte has an absorbance of 0.338 at a wavelength of 490 nm, which corresponds to a molar absorptivity of 676 cm/M

    UV-Vis Molecular Absorption Spectrometry

    • Many types of molecular species absorb ultraviolet and visible radiation, which can be used for qualitative and quantitative analyses
    • Molecular absorption by these species can be used to measure the absorption or emission of electromagnetic radiation by a chemical substance
    • Electromagnetic radiation (EMR) is a form of energy that consists of perpendicular oscillating electric and magnetic fields, which travel in a bundle of energy called a photon

    Electromagnetic Radiation (EMR)

    • EMR has several forms, including X-ray, microwaves, UV, and visible light
    • EMR has several properties, including amplitude, wavelength, frequency, and wave number
    • Amplitude is the maximum vertical displacement of the wave from its normal position
    • Wavelength is a measure of the distance between two identical peaks or troughs in a wave
    • Frequency is the number of waves that pass a fixed point per second, and is measured in Hertz (Hz)
    • Wave number is the number of waves in a unit length or distance per cycle, and is a reciprocal of the wavelength

    Absorption of Light

    • If a sample absorbs all wavelengths in the visible region of the spectrum, it will appear black
    • If a sample absorbs none of the wavelengths in the visible region of the spectrum, it will appear white
    • The color of an object is due to the wavelengths transmitted or reflected, while the other wavelengths are absorbed

    Types of Absorbing Species

    • Organic compounds can absorb light due to electronic transitions, including π → π* and n → π* transitions
    • Saturated compounds containing atoms with unpaired electrons can absorb light due to n → σ* transitions
    • Unsaturated systems with π electrons or n electrons can absorb light due to electronic transitions

    Chromophores

    • Chromophores are unsaturated organic functional groups that absorb in the ultraviolet or visible region
    • Most absorption bands of organic compounds are due to electronic transitions

    Inorganic Compounds

    • Metal-ligand complexes can absorb light due to charge-transfer transitions
    • d-d transitions can occur between d orbitals of metal split by the presence of a ligand field
    • These transitions usually absorb in the visible region of the spectrum

    Applications of Molecular Absorption Spectrometry

    • The determination of an analyte's concentration based on its absorption of ultraviolet or visible radiation is a common quantitative analytical method
    • Many organic and inorganic compounds have strong absorption bands in the UV/Vis region of the electromagnetic spectrum
    • If an analyte does not absorb UV/Vis radiation, it can often be reacted with another species that is strongly absorbing

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    Description

    Practice exercises to calculate absorbance and transmittance in spectroscopy, including solving for percentage transmittance and absorbance.

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