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
