Achem Exam 4 Questions With Complete Solutions
How are absorbance and transmittance mathematically related,
and what does this imply about light passing through a sample?
Correct Answers A = −log(T); as transmittance decreases,
absorbance increases.
How can absorbance measurements at multiple wavelengths be
used to determine concentrations in a mixture? Correct Answers
By writing and solving simultaneous Beer's Law equations.
How can you calculate the energy of a photon from its
wavelength, and what constants are required? Correct Answers
Use E = hc/λ, where h = Planck's constant and c = speed of light.
How do accuracy and precision differ in analytical
measurements? Correct Answers Accuracy is closeness to the
true value; precision is reproducibility.
How do fluorescence and phosphorescence differ in terms of
timing and energy of emitted light? Correct Answers
Fluorescence is immediate and higher energy; phosphorescence
is delayed and lower energy.
How do you calculate molar absorptivity (ε) from a Beer-
Lambert measurement? Correct Answers Rearrange A = εbc to
get ε = A/(bc), with units of M⁻¹cm⁻¹.
How do you calculate the number of theoretical plates (N) from
a chromatogram? Correct Answers N = 16(tᵣ/w)² or N =
, 5.54(tᵣ/w₁/₂)², where w = peak width at base and w₁/₂ = width at
half height.
How does a double-beam spectrophotometer differ from a
single-beam instrument? Correct Answers A double-beam
splits light between sample and reference simultaneously,
correcting for source fluctuations and drift.
How does quality assessment (QA) differ from quality control,
and what does it monitor? Correct Answers QA monitors
whether the system remains under statistical control.
If a solution has an absorbance of 0.450, a path length of 1.00
cm, and a molar absorptivity of 1500 M⁻¹cm⁻¹, what is its
concentration? Correct Answers c = A/(εb) = 0.450/(1500 ×
1.00) = 3.0 × 10⁻⁴ M.
Under what condition will an atom or molecule absorb
electromagnetic radiation? Correct Answers When the photon
energy exactly matches the energy gap (ΔE) between two energy
levels.
What are common causes of deviations from Beer's Law?
Correct Answers High analyte concentrations, chemical
equilibria, stray light, polychromatic radiation, and refractive
index changes.
What are common detectors used in gas chromatography (GC)?
Correct Answers Flame ionization detector (FID), thermal
conductivity detector (TCD), electron capture detector (ECD),
and mass spectrometer (MS).
How are absorbance and transmittance mathematically related,
and what does this imply about light passing through a sample?
Correct Answers A = −log(T); as transmittance decreases,
absorbance increases.
How can absorbance measurements at multiple wavelengths be
used to determine concentrations in a mixture? Correct Answers
By writing and solving simultaneous Beer's Law equations.
How can you calculate the energy of a photon from its
wavelength, and what constants are required? Correct Answers
Use E = hc/λ, where h = Planck's constant and c = speed of light.
How do accuracy and precision differ in analytical
measurements? Correct Answers Accuracy is closeness to the
true value; precision is reproducibility.
How do fluorescence and phosphorescence differ in terms of
timing and energy of emitted light? Correct Answers
Fluorescence is immediate and higher energy; phosphorescence
is delayed and lower energy.
How do you calculate molar absorptivity (ε) from a Beer-
Lambert measurement? Correct Answers Rearrange A = εbc to
get ε = A/(bc), with units of M⁻¹cm⁻¹.
How do you calculate the number of theoretical plates (N) from
a chromatogram? Correct Answers N = 16(tᵣ/w)² or N =
, 5.54(tᵣ/w₁/₂)², where w = peak width at base and w₁/₂ = width at
half height.
How does a double-beam spectrophotometer differ from a
single-beam instrument? Correct Answers A double-beam
splits light between sample and reference simultaneously,
correcting for source fluctuations and drift.
How does quality assessment (QA) differ from quality control,
and what does it monitor? Correct Answers QA monitors
whether the system remains under statistical control.
If a solution has an absorbance of 0.450, a path length of 1.00
cm, and a molar absorptivity of 1500 M⁻¹cm⁻¹, what is its
concentration? Correct Answers c = A/(εb) = 0.450/(1500 ×
1.00) = 3.0 × 10⁻⁴ M.
Under what condition will an atom or molecule absorb
electromagnetic radiation? Correct Answers When the photon
energy exactly matches the energy gap (ΔE) between two energy
levels.
What are common causes of deviations from Beer's Law?
Correct Answers High analyte concentrations, chemical
equilibria, stray light, polychromatic radiation, and refractive
index changes.
What are common detectors used in gas chromatography (GC)?
Correct Answers Flame ionization detector (FID), thermal
conductivity detector (TCD), electron capture detector (ECD),
and mass spectrometer (MS).
