INSTRUMENTAL ANALYSIS STANDARD EXAM SHEET
QUESTIONS AND SOLUTIONS 2025/2026 ALL GRADED A+
✔✔Light intensity - ✔✔-Intensity is like brightness, and is measured as the rate at which
light energy is delivered to a unit of surface, or energy per unit time per unit area.
(photons/area*time unit)
✔✔spectroscopy methods - ✔✔based on interactions of electromagnetic radiation with
matter: includes emission, absorption, scattering
✔✔Qualitative analysis-spectroscopy - ✔✔the region or specific wavelength of
absorption or emission
-V spectrophotometers measure the visible regions of ultraviolet light and can provide
valuable information about the levels of active ingredients present in pharmaceutical
compounds,
✔✔Quantitative analysis-spectroscopy - ✔✔amount of energy(light) absorbed or emitted
✔✔molecular UV-Vis absorbance - ✔✔-In molecular absorbance spectroscopy, a beam
of ultraviolet or visible light is directed through a sample.
-Some of the light may be transmitted through the sample.
-Light that was not transmitted through the sample was absorbed.
✔✔chromophore - ✔✔-an atom or group whose presence is responsible for the color of
a compound.
-Visible light that hits the chromophore can thus be absorbed by exciting an electron
from its ground state into an excited state.
✔✔Energy levels of molecules - ✔✔1) primary=electronic
2) vibrational
3) rotational
✔✔excitation of electrons for absorbance - ✔✔-A molecule can absorb some of the light
only if it can accommodate that additional energy by promoting electrons to higher
energy levels.
✔✔Beer's Law - ✔✔A=abc
where: A is absorbance
a describes the ability of a molecule to absorb radiation at a particular wavelength.
b is the length of sample through which the light beam passes.
c is the concentration of the absorbing species.
✔✔absorbance spectroscopy relationship to Beer's Law - ✔✔-This relationship is the
basis of all quantitative work in absorbance spectroscopy.
, -It allows one to determine the concentration of an absorbing species simply by
measuring its absorbance.
✔✔Selection rules - ✔✔a.k.a. transition rules: formally constrain the possible transitions
of a system from one quantum state to another.
✔✔forbidden transitions - ✔✔-transitions between like atomic orbitals such as s-s,p-p,
d-d, or f-f, transitions are forbidden.
✔✔allowed transitions - ✔✔-must involve the promotion of electrons without a change in
their spin (or the # of unpaired electrons)
✔✔sigma--> sigma star - ✔✔ex: methane, absorbs in a vacuum UV (50-200 nm)
✔✔n --> sigma star - ✔✔ex: 1-chloromethane, absorbs in a vacuum UV (50-200 nm),
sometimes absorbs in the UV
✔✔pi --> pi star - ✔✔ex: ethylene, absorbs in UV-Vis (UV: 200-400 nm, Vis: 400-700
nm)
✔✔n--> pi star - ✔✔ex: carbonyl, absorbs in UV-Vis (UV: 200-400 nm, Vis: 400-700 nm)
✔✔examples of organic molecules not easily detectable in UV-Vis range -
✔✔cyclohexane, hexane
✔✔why is hexane a good solvent for the UV-Vis range? - ✔✔because hexane is not
visible/detectable in the UV-Vis range
✔✔where does cyclohexene absorb? - ✔✔-UV-Vis region because of the C=C, pi --> pi
star region
✔✔what happens when we extend conjugation? - ✔✔-lambda max shifts to longer
wavelength, and the molar absorptivity increases (~207 nm)
✔✔why can't 1,3-cylclohexadiene extend it's conjugation to benzene? - ✔✔because
benzene is aromatic, absorbs ~257 nm
✔✔molar absorptivity - ✔✔-The molar absorption coefficient is a measurement of how
strongly a substance absorbs light.
-The larger its value, the greater the absorption.
✔✔conjugation - ✔✔-When a compound is conjugated, the energy gap between the
ground state level and excited state level or the HOMO-LUMO gap decreases. Due to
this the wavelength of the light absorbed becomes longer.
QUESTIONS AND SOLUTIONS 2025/2026 ALL GRADED A+
✔✔Light intensity - ✔✔-Intensity is like brightness, and is measured as the rate at which
light energy is delivered to a unit of surface, or energy per unit time per unit area.
(photons/area*time unit)
✔✔spectroscopy methods - ✔✔based on interactions of electromagnetic radiation with
matter: includes emission, absorption, scattering
✔✔Qualitative analysis-spectroscopy - ✔✔the region or specific wavelength of
absorption or emission
-V spectrophotometers measure the visible regions of ultraviolet light and can provide
valuable information about the levels of active ingredients present in pharmaceutical
compounds,
✔✔Quantitative analysis-spectroscopy - ✔✔amount of energy(light) absorbed or emitted
✔✔molecular UV-Vis absorbance - ✔✔-In molecular absorbance spectroscopy, a beam
of ultraviolet or visible light is directed through a sample.
-Some of the light may be transmitted through the sample.
-Light that was not transmitted through the sample was absorbed.
✔✔chromophore - ✔✔-an atom or group whose presence is responsible for the color of
a compound.
-Visible light that hits the chromophore can thus be absorbed by exciting an electron
from its ground state into an excited state.
✔✔Energy levels of molecules - ✔✔1) primary=electronic
2) vibrational
3) rotational
✔✔excitation of electrons for absorbance - ✔✔-A molecule can absorb some of the light
only if it can accommodate that additional energy by promoting electrons to higher
energy levels.
✔✔Beer's Law - ✔✔A=abc
where: A is absorbance
a describes the ability of a molecule to absorb radiation at a particular wavelength.
b is the length of sample through which the light beam passes.
c is the concentration of the absorbing species.
✔✔absorbance spectroscopy relationship to Beer's Law - ✔✔-This relationship is the
basis of all quantitative work in absorbance spectroscopy.
, -It allows one to determine the concentration of an absorbing species simply by
measuring its absorbance.
✔✔Selection rules - ✔✔a.k.a. transition rules: formally constrain the possible transitions
of a system from one quantum state to another.
✔✔forbidden transitions - ✔✔-transitions between like atomic orbitals such as s-s,p-p,
d-d, or f-f, transitions are forbidden.
✔✔allowed transitions - ✔✔-must involve the promotion of electrons without a change in
their spin (or the # of unpaired electrons)
✔✔sigma--> sigma star - ✔✔ex: methane, absorbs in a vacuum UV (50-200 nm)
✔✔n --> sigma star - ✔✔ex: 1-chloromethane, absorbs in a vacuum UV (50-200 nm),
sometimes absorbs in the UV
✔✔pi --> pi star - ✔✔ex: ethylene, absorbs in UV-Vis (UV: 200-400 nm, Vis: 400-700
nm)
✔✔n--> pi star - ✔✔ex: carbonyl, absorbs in UV-Vis (UV: 200-400 nm, Vis: 400-700 nm)
✔✔examples of organic molecules not easily detectable in UV-Vis range -
✔✔cyclohexane, hexane
✔✔why is hexane a good solvent for the UV-Vis range? - ✔✔because hexane is not
visible/detectable in the UV-Vis range
✔✔where does cyclohexene absorb? - ✔✔-UV-Vis region because of the C=C, pi --> pi
star region
✔✔what happens when we extend conjugation? - ✔✔-lambda max shifts to longer
wavelength, and the molar absorptivity increases (~207 nm)
✔✔why can't 1,3-cylclohexadiene extend it's conjugation to benzene? - ✔✔because
benzene is aromatic, absorbs ~257 nm
✔✔molar absorptivity - ✔✔-The molar absorption coefficient is a measurement of how
strongly a substance absorbs light.
-The larger its value, the greater the absorption.
✔✔conjugation - ✔✔-When a compound is conjugated, the energy gap between the
ground state level and excited state level or the HOMO-LUMO gap decreases. Due to
this the wavelength of the light absorbed becomes longer.
