Beer–Lambert’s law MCQs With Answer

Beer–Lambert’s law MCQs With Answer

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Beer–Lambert’s law MCQs With Answer help M. Pharm students strengthen their command over quantitative UV–Visible spectrophotometry, a cornerstone of Modern Pharmaceutical Analytical Techniques. This quiz set explores core equations and definitions, instrument and method variables, and real-world limitations such as stray light, polychromatic radiation, matrix effects, and chemical deviations. You will practice interpreting transmittance–absorbance relationships, selecting optimal wavelengths and cuvettes, handling calibration strategies (external, matrix-matched, standard addition), and solving quick calculation problems using molar absorptivity. The questions also touch on multi-component analysis and best practices for precision. Work through them to bridge theory with analytical decision-making in pharmaceutical assay development and validation.

Q1. Which mathematical form correctly expresses the Beer–Lambert law for a single absorbing species?

  • A = ε b c
  • A = ε b / c
  • A = ε c / b
  • A = ε + b + c

Correct Answer: A = ε b c

Q2. The SI-consistent units commonly used for decadic molar absorptivity (ε) in UV–Vis spectrophotometry are:

  • L mol^-1 cm^-1
  • cm^2 mol^-1
  • mol L^-1 cm
  • AU

Correct Answer: L mol^-1 cm^-1

Q3. A solution shows 20% transmittance at a given wavelength. What is its absorbance (base 10)?

  • 0.699
  • 0.301
  • 1.000
  • 0.200

Correct Answer: 0.699

Q4. Which of the following is NOT an assumption/requirement for strict Beer–Lambert linearity?

  • The solution is perfectly scattering
  • The incident radiation is monochromatic
  • The absorbing species do not associate, dissociate, or chemically interact
  • The medium is optically homogeneous

Correct Answer: The solution is perfectly scattering

Q5. If the path length is doubled while concentration and ε remain constant, how does absorbance change?

  • Absorbance doubles
  • Absorbance halves
  • Absorbance increases by √2
  • Absorbance remains unchanged

Correct Answer: Absorbance doubles

Q6. What is a primary cause of chemical deviation from Beer–Lambert law at higher concentrations?

  • Solute–solute interactions altering effective ε
  • Detector dark current drift
  • Increased lamp intensity
  • Variation of cuvette path length with wavelength

Correct Answer: Solute–solute interactions altering effective ε

Q7. How does stray light affect absorbance readings, particularly at high true absorbance?

  • Apparent absorbance is lower than true, causing downward curvature at high A
  • Apparent absorbance is higher than true, causing upward curvature
  • Stray light has no measurable effect on absorbance
  • Stray light only shifts the λmax without changing A

Correct Answer: Apparent absorbance is lower than true, causing downward curvature at high A

Q8. To maximize sensitivity for a chromophore in UV–Vis, you should measure absorbance:

  • At or very close to λmax where ε is highest
  • At a valley where ε is minimal
  • At any wavelength in the near-IR region
  • At the isosbestic point of an unrelated system

Correct Answer: At or very close to λmax where ε is highest

Q9. For a mixture of non-interacting species, the Beer–Lambert law implies that:

  • Absorbance of a mixture at a wavelength equals the sum of individual absorbances
  • Absorbance of a mixture equals the product of individual absorbances
  • Absorbance of a mixture is independent of concentration
  • Beer–Lambert law cannot be applied to mixtures

Correct Answer: Absorbance of a mixture at a wavelength equals the sum of individual absorbances

Q10. A solution has ε = 15000 L mol^-1 cm^-1 at 280 nm, b = 1.0 cm, and c = 2.0 × 10^-5 mol L^-1. What is A?

  • 0.300
  • 0.030
  • 3.000
  • 0.015

Correct Answer: 0.300

Q11. A substance has A(1%, 1 cm) = 200 at 280 nm. If a 1 cm cell gives A = 1.0, what is the sample concentration in % w/v?

  • 0.005% w/v
  • 0.5% w/v
  • 0.02% w/v
  • 2.0% w/v

Correct Answer: 0.005% w/v

Q12. The relationship between Napierian (natural log) and decadic molar absorptivity is:

  • ε_n = 2.303 ε_10
  • ε_n = ε_10 / 2.303
  • ε_n = ε_10
  • ε_n = 10 ε_10

Correct Answer: ε_n = 2.303 ε_10

Q13. What is the effect of using an excessively large spectral bandwidth (slit width) relative to the analyte’s absorption band?

  • Polychromatic light yields lower apparent ε and negative deviation from linearity
  • It increases absorbance uniformly without affecting linearity
  • It causes positive deviation at low absorbance only
  • It merely broadens the baseline with no quantitative impact

Correct Answer: Polychromatic light yields lower apparent ε and negative deviation from linearity

Q14. Which cuvette material is appropriate for measurements in the deep UV region (e.g., 200–300 nm)?

  • Quartz or fused silica cuvette
  • Soda-lime glass cuvette
  • Plastic cuvette designed for visible range
  • Ceramic cuvette

Correct Answer: Quartz or fused silica cuvette

Q15. A Beer’s plot (A vs. c) shows a significant non-zero intercept near zero concentration. A likely cause is:

  • Instrument baseline offset or stray light
  • A perfectly matched solvent blank
  • Absolute chemical purity of analyte
  • Infinite dilution conditions

Correct Answer: Instrument baseline offset or stray light

Q16. If A = 2.0 at a given wavelength, what is the percent transmittance (%T)?

  • 1%
  • 10%
  • 0.1%
  • 50%

Correct Answer: 1%

Q17. When quantifying an analyte in a complex matrix with potential interferences, which calibration strategy best reduces matrix effects?

  • Standard addition method
  • External standard method without matrix matching
  • Single-point calibration at any convenient concentration
  • Using distilled water as blank regardless of sample composition

Correct Answer: Standard addition method

Q18. In two-component analysis using absorbances at two wavelengths, which condition is necessary for unique concentration solutions?

  • The absorptivity coefficients at the two wavelengths must be sufficiently different (linearly independent)
  • Both components must have identical spectra
  • The path length must be unknown
  • The concentrations must be equal

Correct Answer: The absorptivity coefficients at the two wavelengths must be sufficiently different (linearly independent)

Q19. For best photometric precision in UV–Vis, the recommended working absorbance range is approximately:

  • 0.2–0.8 A
  • 0.01–0.05 A
  • 1.5–3.0 A
  • 0.9–1.2 A

Correct Answer: 0.2–0.8 A

Q20. If the sample fluoresces or strongly scatters at the measurement wavelength, what is the effect on the apparent absorbance?

  • Emission or scattering increases transmitted light, lowering apparent absorbance
  • It has no effect because the detector distinguishes light types perfectly
  • It raises apparent absorbance by reducing transmitted light
  • It narrows the spectral bandwidth automatically

Correct Answer: Emission or scattering increases transmitted light, lowering apparent absorbance

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