Mean particle size MCQs With Answer

Introduction

Mean particle size MCQs with answer are essential for B. Pharm students studying pharmaceutics, particle size distribution and its effects on dissolution, stability, and bioavailability. Understanding mean particle size, including number mean, surface mean (D[3,2]), and volume mean (D[4,3]), plus methods like sieving, laser diffraction and microscopy, helps predict formulation performance. Mastery of calculation formulas, polydispersity, span and parameters such as settling velocity (Stokes’ law) or specific surface area is crucial for drug design, quality control and regulatory compliance. This set of targeted multiple-choice questions reinforces theoretical concepts and practical calculations in particle sizing and analysis. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What does the term “mean particle size” commonly refer to in pharmaceutics?

• The average dimension of particles in a sample
• The largest particle present in a formulation
• The density of particles
• The chemical composition of particles

Correct Answer: The average dimension of particles in a sample

Q2. Which mean diameter represents the ratio of the third moment to the second moment (volume-to-surface) and is sensitive to large particles?

• Number mean diameter (D1)
• Surface mean diameter D[3,2]
• Volume mean diameter D[4,3]
• Median diameter D50

Correct Answer: Volume mean diameter D[4,3]

Q3. Which mean diameter emphasizes small particles because it is based on particle surface area?

• Volume mean diameter D[4,3]
• Number mean diameter (Dn)
• Surface mean diameter D[3,2]
• Mode diameter

Correct Answer: Surface mean diameter D[3,2]

Q4. In sieve analysis, which parameter gives the particle size below which 50% of the sample lies?

• Mode
• Mean
• D50 (median diameter)
• Span

Correct Answer: D50 (median diameter)

Q5. Which technique measures particle size distribution by analyzing light scattering intensity as a function of angle?

• Sieving
• Laser diffraction
• Tap density
• Conductivity measurement

Correct Answer: Laser diffraction

Q6. Stokes’ law relates settling velocity to which of the following factors?

• Particle color and refractive index
• Particle size, density difference, fluid viscosity, and gravity
• Chemical reactivity and surface charge
• Temperature only

Correct Answer: Particle size, density difference, fluid viscosity, and gravity

Q7. For a polydisperse powder, which metric describes the breadth of the particle size distribution?

• Median alone
• Span
• Color index
• True density

Correct Answer: Span

Q8. How is the span of a distribution commonly calculated?

• (D10 + D90) / D50
• (D90 – D10) / D50
• D50 / (D90 – D10)
• D10 × D90 × D50

Correct Answer: (D90 – D10) / D50

Q9. Which mean is most appropriate when counting particles individually by microscopy?

• Volume mean D[4,3]
• Surface mean D[3,2]
• Number mean diameter (Dn)
• Mass median aerodynamic diameter (MMAD)

Correct Answer: Number mean diameter (Dn)

Q10. Which measurement gives information about specific surface area relevant to dissolution rate?

• Number mean diameter only
• Surface area per unit mass
• Porosity only
• Tapped density only

Correct Answer: Surface area per unit mass

Q11. A batch has D10 = 5 µm, D50 = 20 µm and D90 = 80 µm. What is the span?

• 3.75
• 0.375
• 4.0
• 2.0

Correct Answer: 3.75

Q12. Which particle sizing method provides number-based distributions and visual morphology?

• Laser diffraction
• Optical microscopy / image analysis
• Gas adsorption
• Mercury porosimetry

Correct Answer: Optical microscopy / image analysis

Q13. In laser diffraction, intensity of scattered light at small angles is most influenced by which particles?

• Fine particles
• Large particles
• Only spherical particles
• Only agglomerates

Correct Answer: Large particles

Q14. Which mean diameter would be most affected by a small number of very large particles?

• Number mean diameter
• Surface mean diameter D[3,2]
• Volume mean diameter D[4,3]
• Median D50

Correct Answer: Volume mean diameter D[4,3]

Q15. The equation D[3,2] = Σ(n_i d_i^3)/Σ(n_i d_i^2) defines which mean?

• Number mean
• Surface mean
• Volume mean
• Harmonic mean

Correct Answer: Surface mean

Q16. Which parameter indicates the uniformity of particle size and is important for content uniformity in tablets?

• Polydispersity index or span
• Melting point
• pH only
• Optical rotation

Correct Answer: Polydispersity index or span

Q17. What does D[4,3] mathematically represent?

• Σ(n_i d_i)/Σ(n_i)
• Σ(n_i d_i^3)/Σ(n_i d_i^2)
• Σ(n_i d_i^4)/Σ(n_i d_i^3)
• Σ(n_i d_i^2)/Σ(n_i d_i)

Correct Answer: Σ(n_i d_i^4)/Σ(n_i d_i^3)

Q18. Which instrument/method is most suitable for nanoparticles (<100 nm) in suspension?

• Standard sieve shaker
• Dynamic light scattering (DLS)
• Tap density tester
• Optical sieve analysis

Correct Answer: Dynamic light scattering (DLS)

Q19. Why is mean particle size critical for dissolution rate of poorly soluble drugs?

• Larger particles increase surface area and speed dissolution
• Smaller particles increase specific surface area and generally enhance dissolution rate
• Particle size has no effect on dissolution
• Dissolution depends only on pH

Correct Answer: Smaller particles increase specific surface area and generally enhance dissolution rate

Q20. Which of the following will NOT change the measured mean particle size by laser diffraction?

• Assumed optical refractive index
• Sample dispersion state (agglomeration)
• Instrument geometry and detection range
• Packaging color of the sample container

Correct Answer: Packaging color of the sample container

Q21. Which statistical mean is most appropriate when mass-weighted properties are important?

• Number mean
• Surface mean D[3,2]
• Volume or mass mean D[4,3]
• Mode

Correct Answer: Volume or mass mean D[4,3]

Q22. If particle density increases while size remains constant, what happens to settling velocity according to Stokes’ law?

• Settling velocity decreases
• Settling velocity remains unchanged
• Settling velocity increases
• Settling velocity becomes zero

Correct Answer: Settling velocity increases

Q23. Which preparation step can reduce mean particle size to improve dissolution?

• Granulation without milling
• Milling or micronization
• Freeze-drying without comminution
• Coating with hydrophobic polymer

Correct Answer: Milling or micronization

Q24. Which of the following best describes polydispersity index (PDI) in DLS output?

• A measure of particle sphericity
• A measure of distribution width; lower PDI indicates more monodisperse sample
• A direct measurement of zeta potential
• Equivalent to tapped density

Correct Answer: A measure of distribution width; lower PDI indicates more monodisperse sample

Q25. In sieve analysis, which result indicates a coarser powder?

• Higher percentage retained on coarse sieve fractions
• Lower percentage retained on coarse sieve fractions
• Identical percent retained across all sieves
• Zero retention on all sieves

Correct Answer: Higher percentage retained on coarse sieve fractions

Q26. Which parameter is most relevant when designing inhalation aerosols for pulmonary deposition?

• Number mean diameter only
• Mass median aerodynamic diameter (MMAD)
• Color and odor
• Melting point

Correct Answer: Mass median aerodynamic diameter (MMAD)

Q27. When converting number distribution to volume distribution, what happens to the relative contribution of large particles?

• Decreases
• Remains the same
• Increases significantly
• Becomes negative

Correct Answer: Increases significantly

Q28. What is one limitation of optical microscopy for particle sizing?

• Cannot provide morphological details
• Has difficulty resolving submicron particles and can be time-consuming
• Always gives volume-weighted distributions
• Destroys the sample chemically

Correct Answer: Has difficulty resolving submicron particles and can be time-consuming

Q29. Which of the following affects powder flow and compressibility most directly related to particle size?

• Particle size and shape
• Only chemical impurities
• Color of powder
• Electrical conductivity

Correct Answer: Particle size and shape

Q30. For quality control, which practice ensures reproducible mean particle size measurements?

• Changing dispersion protocol each run
• Standardizing sample preparation, dispersion and instrument settings
• Using different instruments without calibration
• Varying refractive index assumption randomly

Correct Answer: Standardizing sample preparation, dispersion and instrument settings

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com