Dissolution Rate & Noyes-Whitney Equation MCQs With Answer

Dissolution Rate & Noyes-Whitney Equation MCQs With Answer

This quiz set focuses on the dissolution rate and the Noyes-Whitney equation—core concepts in Advanced Biopharmaceutics & Pharmacokinetics (MIP 201T). It covers the mathematical form of the equation, interpretation of its parameters (diffusion coefficient, surface area, diffusion layer thickness, saturation solubility and bulk concentration), assumptions and limitations, intrinsic dissolution, experimental methods, and implications for formulation and IVIVC. These MCQs are designed to deepen understanding beyond definitions—emphasizing how formulation variables, hydrodynamics, and solubility modify dissolution behavior and how to apply the Noyes-Whitney framework when predicting and optimizing drug release and absorption.

Q1. Which of the following correctly expresses the Noyes-Whitney equation for the dissolution rate (dM/dt)?

• dM/dt = D·A·(Cs – C)·h
• dM/dt = (D·A·(Cs – C))/h
• dM/dt = (D·h·(Cs – C))/A
• dM/dt = (A·h)/(D·(Cs – C))

Correct Answer: dM/dt = (D·A·(Cs – C))/h

Q2. In the Noyes-Whitney equation, what does D represent?

• Diffusion layer thickness
• Diffusion coefficient of the drug in the dissolution medium
• Drug solubility at the particle surface
• Surface area of the dissolution vessel

Correct Answer: Diffusion coefficient of the drug in the dissolution medium

Q3. What is Cs in the context of the Noyes-Whitney equation?

• Bulk concentration of dissolved drug
• Saturation solubility at the solid surface (surface concentration)
• Overall concentration in dosing formulation
• Critical supersaturation value

Correct Answer: Saturation solubility at the solid surface (surface concentration)

Q4. Which parameter in the Noyes-Whitney equation represents the thickness of the stagnant diffusion layer?

• A — surface area
• D — diffusion coefficient
• h — diffusion layer thickness
• Cs — saturation solubility

Correct Answer: h — diffusion layer thickness

Q5. Which formulation strategy will most effectively increase the dissolution rate according to the Noyes-Whitney equation?

• Decrease diffusion coefficient (D)
• Increase diffusion layer thickness (h)
• Reduce particle size to increase surface area (A)
• Lower saturation solubility (Cs)

Correct Answer: Reduce particle size to increase surface area (A)

Q6. Under sink conditions, which relationship between bulk concentration (C) and surface concentration (Cs) is ensured?

• C is approximately equal to Cs
• C is greater than Cs
• C is much less than Cs (usually < ~10% of Cs)
• C is irrelevant under sink conditions

Correct Answer: C is much less than Cs (usually < ~10% of Cs)

Q7. Intrinsic dissolution rate is defined as:

• The dissolution rate measured per unit mass of drug
• The dissolution rate per unit surface area under constant conditions
• The total amount dissolved after a fixed time
• The dissolution rate in presence of surfactant only

Correct Answer: The dissolution rate per unit surface area under constant conditions

Q8. Which experimental method is most commonly used to measure intrinsic dissolution?

• USP paddle method with free powder
• Rotating disk method (intrinsic dissolution apparatus)
• Capsule dissolution test
• UV photometric scanning of powders

Correct Answer: Rotating disk method (intrinsic dissolution apparatus)

Q9. According to the Noyes-Whitney equation, increasing agitation of the dissolution medium primarily affects which variable?

• Increases Cs (saturation solubility)
• Decreases diffusion layer thickness (h)
• Reduces surface area (A)
• Decreases diffusion coefficient (D)

Correct Answer: Decreases diffusion layer thickness (h)

Q10. Which assumption is NOT part of the classical Noyes-Whitney description?

• A stagnant diffusion layer exists at the solid surface
• Concentration at the solid surface equals saturation solubility (Cs)
• The bulk solution is instantaneously and uniformly mixed to maintain sink conditions
• The equation explicitly includes pH as a separate variable

Correct Answer: The equation explicitly includes pH as a separate variable

Q11. The Noyes-Whitney equation can be derived from which fundamental law of mass transport?

• Fick’s first law of diffusion
• Raoult’s law
• Henry’s law
• Arrhenius equation

Correct Answer: Fick’s first law of diffusion

Q12. For a perfect sphere, the surface area of a single particle is given by which expression?

• 4/3·π·r^3
• π·r^2
• 4·π·r^2
• 2·π·r

Correct Answer: 4·π·r^2

Q13. Which unit is appropriate for the diffusion coefficient (D) in the Noyes-Whitney equation when using cm and s?

• cm/s
• cm^2/s
• mg·cm^-3
• cm^3/s

Correct Answer: cm^2/s

Q14. Consider the Noyes-Whitney equation. If the diffusion layer thickness (h) is halved while other factors remain constant, the dissolution rate will:

• Remain unchanged
• Be halved
• Double
• Increase by fourfold

Correct Answer: Double

Q15. Which of the following is a limitation of the Noyes-Whitney equation when applied to real dosage forms?

• It accounts for changes in particle aggregation over time
• It assumes constant surface area and constant Cs during dissolution
• It explicitly models convective mass transfer in stirred vessels
• It integrates metabolism occurring during dissolution

Correct Answer: It assumes constant surface area and constant Cs during dissolution

Q16. Simple calculation: For a flat disk with surface area A = 1.0 cm^2, diffusion coefficient D = 1.0×10^-5 cm^2/s, Cs – C = 1.0 mg/cm^3 and h = 0.1 cm, what is dM/dt (mg/s) according to Noyes-Whitney?

• 1.0×10^-3 mg/s
• 1.0×10^-4 mg/s
• 1.0×10^-5 mg/s
• 1.0×10^-6 mg/s

Correct Answer: 1.0×10^-4 mg/s

Q17. Which condition most often makes dissolution the rate-limiting step for oral absorption?

• High permeability and high aqueous solubility (BCS Class I)
• Low permeability and high solubility (BCS Class III)
• High permeability and low solubility (BCS Class II)
• Low permeability and low solubility (BCS Class IV)

Correct Answer: High permeability and low solubility (BCS Class II)

Q18. How does particle aggregation typically affect dissolution rate predicted by Noyes-Whitney?

• Aggregation increases exposed surface area, increasing rate
• Aggregation reduces effective surface area, decreasing rate
• Aggregation has no effect on dissolution
• Aggregation directly increases diffusion coefficient (D)

Correct Answer: Aggregation reduces effective surface area, decreasing rate

Q19. Which experimental change would most directly increase Cs (saturation solubility) of a poorly soluble drug in vitro?

• Reduce temperature of the medium
• Add a co-solvent or surfactant to the medium
• Decrease agitation rate
• Compact the drug into larger particles

Correct Answer: Add a co-solvent or surfactant to the medium

Q20. How is the Noyes-Whitney approach useful in developing IVIVC (in vitro–in vivo correlation)?

• It replaces the need for any in vivo studies
• It provides mechanistic linkage between in vitro dissolution rate and in vivo absorption for dissolution-limited drugs
• It directly predicts metabolic clearance from dissolution data
• It shows that permeability is always the limiting factor regardless of solubility

Correct Answer: It provides mechanistic linkage between in vitro dissolution rate and in vivo absorption for dissolution-limited drugs

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