Distribution law MCQs With Answer

Distribution law MCQs With Answer

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Distribution law MCQs With Answer is an essential topic for B.Pharm students studying drug partitioning, extraction, and formulation. This introduction offers clear, exam-focused practice on the Nernst distribution law, distribution coefficient (K), partition coefficient (P), and pH-dependent distribution (D). You’ll revise fundamental principles, assumptions, common deviations (association, ion-pairing), measurement methods (shake-flask, octanol-water), and applications in drug extraction, absorption, and formulation design. Each question emphasizes calculation skills, conceptual understanding, and real-world pharmaceutical implications to boost practical competence. Targeted keywords included: Distribution law MCQs With Answer, distribution coefficient, partition coefficient, Nernst law, lipophilicity, and pH-partition. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What does the Nernst distribution law describe?

  • The equilibrium ratio of a solute’s concentrations between two immiscible solvents
  • The rate of solute diffusion through a membrane
  • The solubility limit of a solute in a single solvent
  • The pH at which a drug is 50% ionized

Correct Answer: The equilibrium ratio of a solute’s concentrations between two immiscible solvents

Q2. Which expression represents the distribution coefficient (K) according to the Nernst law?

  • K = concentration in solvent A × concentration in solvent B
  • K = concentration in organic phase / concentration in aqueous phase
  • K = molecular weight of solute / volume of solvent
  • K = solubility in water – solubility in organic solvent

Correct Answer: K = concentration in organic phase / concentration in aqueous phase

Q3. Under which condition is the Nernst distribution law strictly valid?

  • When solute forms dimers in one phase
  • When the solute is chemically identical in both phases and no association occurs
  • When pH varies between the two phases
  • When the solute ionizes extensively in both phases

Correct Answer: When the solute is chemically identical in both phases and no association occurs

Q4. Which solvent system is most commonly used to determine lipophilicity (log P) in pharmaceutics?

  • Hexane–water
  • Chloroform–methanol
  • n-Octanol–water
  • Benzene–water

Correct Answer: n-Octanol–water

Q5. How does pH affect the distribution of an ionizable drug?

  • pH has no effect on distribution
  • Only affects distribution if the drug is nonpolar
  • Changes the degree of ionization, altering distribution between phases
  • Only changes K when temperature is high

Correct Answer: Changes the degree of ionization, altering distribution between phases

Q6. What is the difference between partition coefficient (P) and distribution coefficient (D)?

  • P is pH-dependent, D is pH-independent
  • P refers to non-ionized solute only; D accounts for ionized and non-ionized species at a given pH
  • P measures solubility; D measures viscosity
  • P is used only for proteins; D is used for small molecules

Correct Answer: P refers to non-ionized solute only; D accounts for ionized and non-ionized species at a given pH

Q7. If a neutral drug has P = 100 (in octanol/water), what general property does this indicate?

  • Very hydrophilic
  • Very lipophilic
  • Completely insoluble in octanol
  • High ionization in water

Correct Answer: Very lipophilic

Q8. Which assumption is NOT part of the ideal Nernst distribution law?

  • No chemical reaction or association in either phase
  • Temperature is constant during equilibrium
  • Concentration measured as activity is identical to analytical concentration without correction
  • The solute must be a gas

Correct Answer: The solute must be a gas

Q9. For a weak acid HA with pKa 5.0, at pH 7.0 most of the drug will be:

  • Non-ionized
  • Ionized (A–)
  • Converted to a base
  • Precipitated

Correct Answer: Ionized (A–)

Q10. Which equation is used to relate ionization to pH for weak acids?

  • Arrhenius equation
  • Henderson-Hasselbalch equation
  • Van’t Hoff equation
  • Michaelis–Menten equation

Correct Answer: Henderson-Hasselbalch equation

Q11. In the shake-flask method, what is primarily measured to obtain K or P?

  • Solid melting point
  • Equilibrium concentrations of solute in each phase
  • Viscosity of each solvent
  • Electrical conductivity

Correct Answer: Equilibrium concentrations of solute in each phase

Q12. If the concentration of drug in octanol is 20 mg/L and in water is 5 mg/L, what is K?

  • 0.25
  • 4
  • 100
  • 5

Correct Answer: 4

Q13. Which factor can cause deviation from the ideal Nernst distribution law?

  • Complete miscibility of solvents
  • Solute association or dimerization in one phase
  • Use of identical solvents for both phases
  • Measuring at equilibrium

Correct Answer: Solute association or dimerization in one phase

Q14. How does temperature generally affect distribution coefficient K?

  • Temperature never affects K
  • K may change because solubility and interactions in each phase are temperature dependent
  • K always increases with temperature for all solutes
  • K becomes zero at high temperature

Correct Answer: K may change because solubility and interactions in each phase are temperature dependent

Q15. The distribution ratio D at a given pH is defined as:

  • Total concentration in organic phase / total concentration in aqueous phase (including all ionization forms)
  • Molecular weight ratio between phases
  • Volume of organic phase / volume of aqueous phase
  • pKa × pH

Correct Answer: Total concentration in organic phase / total concentration in aqueous phase (including all ionization forms)

Q16. For a weak base B with pKa 8.0, at pH 6.0 what form predominates?

  • Non-ionized base (B)
  • Ionized form (BH+)
  • Neutral acid (HA)
  • Anionic dimer

Correct Answer: Ionized form (BH+)

Q17. When extracting an acid from aqueous phase into organic phase, which pH condition favors extraction of the unionized form?

  • pH much lower than pKa for an acid
  • pH equal to pKa
  • pH much higher than pKa for an acid
  • pH has no role

Correct Answer: pH much lower than pKa for an acid

Q18. If K = 10 and equal volumes of aqueous and organic are used, fraction of drug in organic phase at equilibrium is approximately:

  • 10/11 (~0.909)
  • 1/11 (~0.091)
  • 0.5
  • 0.1

Correct Answer: 10/11 (~0.909)

Q19. Which of the following improves the efficiency of multiple solvent extractions compared to a single large-volume extraction?

  • Using one large extraction is always better
  • Using several smaller extractions with fresh solvent each time
  • Increasing temperature only
  • Keeping phases unmixed

Correct Answer: Using several smaller extractions with fresh solvent each time

Q20. If initial amount of drug in aqueous phase is 100 mg, K = 4, and equal volumes are used, amount in organic phase at equilibrium is:

  • 20 mg
  • 80 mg
  • 50 mg
  • 4 mg

Correct Answer: 80 mg

Q21. Which statement about log P is correct?

  • log P is the base-10 logarithm of the partition coefficient for the non-ionized form
  • log P is unrelated to lipophilicity
  • log P is always negative for lipophilic drugs
  • log P measures aqueous solubility only

Correct Answer: log P is the base-10 logarithm of the partition coefficient for the non-ionized form

Q22. Why is activity sometimes preferred over analytical concentration in rigorous thermodynamic treatment of distribution?

  • Activity corrects for non-ideal behavior and ionic strength effects
  • Activity is easier to measure
  • Concentration cannot be used in any equilibrium law
  • Activity ignores temperature effects

Correct Answer: Activity corrects for non-ideal behavior and ionic strength effects

Q23. In which scenario will D (distribution ratio) equal P (partition coefficient)?

  • When the solute is fully ionized at the given pH
  • When the solute is non-ionizable at the given pH
  • When temperature is zero
  • When solvents are miscible

Correct Answer: When the solute is non-ionizable at the given pH

Q24. What is an important pharmaceutical application of the distribution law?

  • Determination of tablet coating hardness
  • Optimizing solvent extraction for purification and formulation design
  • Measuring blood pressure
  • Calculating capsule volume

Correct Answer: Optimizing solvent extraction for purification and formulation design

Q25. If a solute forms an ion-pair in the aqueous phase, how does that affect apparent distribution?

  • Ion-pairing can increase apparent transfer to organic phase, altering observed K
  • Ion-pairing always decreases transfer to organic phase
  • Ion-pairing has no effect on distribution
  • Ion-pairing converts organic solvent into aqueous

Correct Answer: Ion-pairing can increase apparent transfer to organic phase, altering observed K

Q26. Which measurement technique can estimate log P without direct shake-flask partitioning?

  • UV-visible spectroscopy of pure solvent only
  • Reverse-phase HPLC calibrated with standards
  • Osmometry
  • Electric conductivity titration

Correct Answer: Reverse-phase HPLC calibrated with standards

Q27. For a weak acid, as pH increases above pKa, what happens to D (octanol/water)?

  • D increases because ionized form partitions favorably into octanol
  • D decreases because ionized form prefers aqueous phase
  • D remains constant
  • D becomes negative

Correct Answer: D decreases because ionized form prefers aqueous phase

Q28. Which of the following best describes “pH-partition hypothesis” in drug absorption?

  • Ionized form always crosses membranes faster than non-ionized
  • Non-ionized, lipophilic form crosses biological membranes more readily than ionized form
  • pH has no influence on membrane passage
  • Only proteins cross membranes, not small molecules

Correct Answer: Non-ionized, lipophilic form crosses biological membranes more readily than ionized form

Q29. A drug with very high log P may present which pharmaceutical challenge?

  • Excessive aqueous solubility
  • Poor aqueous solubility and formulation difficulty
  • Inability to bind to proteins
  • Always rapid elimination

Correct Answer: Poor aqueous solubility and formulation difficulty

Q30. When a solute associates to form dimers in the organic phase, observed K will be:

  • Unaffected
  • Lower than expected based on monomer concentrations
  • Exactly equal to 1
  • Infinite

Correct Answer: Lower than expected based on monomer concentrations

Q31. In extraction calculations, fraction remaining in the aqueous phase after one extraction is given by:

  • 1 / (1 + K × (Vorg/Vaq))
  • K × (Vorg/Vaq)
  • 1 − K
  • K / (1 + K)

Correct Answer: 1 / (1 + K × (Vorg/Vaq))

Q32. For repeated extraction, the fraction remaining after n identical extractions is:

  • [1 / (1 + K × (Vorg/Vaq))]^n
  • n / (1 + K)
  • K^n
  • 1 − [1 / (1 + K × (Vorg/Vaq))]

Correct Answer: [1 / (1 + K × (Vorg/Vaq))]^n

Q33. Which property correlates most directly with membrane permeability and drug absorption?

  • Atomic number
  • Lipophilicity (log P/D)
  • pKa only
  • Color of the compound

Correct Answer: Lipophilicity (log P/D)

Q34. If a solute has K = 0.01 (octanol/water), what does this mean?

  • Solute strongly prefers octanol
  • Solute strongly prefers water
  • Solute is equally distributed
  • K value indicates complete ionization

Correct Answer: Solute strongly prefers water

Q35. Which of the following is true about measuring distribution coefficients for ionizable drugs?

  • Measure D at only one arbitrary pH gives complete picture
  • D should be measured across a pH range to map pH-dependent behavior
  • pH control is unnecessary
  • Ionizable drugs cannot be measured by partition methods

Correct Answer: D should be measured across a pH range to map pH-dependent behavior

Q36. In chromatography, retention in reverse-phase systems is often correlated with:

  • Log P or lipophilicity of analyte
  • The taste of the compound
  • Molecular chirality only
  • Number of hydrogen atoms exclusively

Correct Answer: Log P or lipophilicity of analyte

Q37. How does salt formation of a drug affect distribution into an organic solvent?

  • Salt formation generally increases partitioning into organic phase
  • Salt formation often increases aqueous solubility and reduces organic partitioning
  • Salt formation converts octanol into water
  • Salt formation has no effect

Correct Answer: Salt formation often increases aqueous solubility and reduces organic partitioning

Q38. Which experimental precaution is important when determining K by shake-flask?

  • Ensure complete equilibrium by sufficient mixing and contact time
  • Remove entire aqueous phase before measuring organic concentration
  • Use any miscible solvents for convenience
  • Ignore temperature control

Correct Answer: Ensure complete equilibrium by sufficient mixing and contact time

Q39. What is a practical reason to determine distribution coefficients in drug development?

  • To decide capsule color
  • To predict absorption, distribution, and extraction behavior during formulation and purification
  • To measure melting point
  • To set refrigeration temperature

Correct Answer: To predict absorption, distribution, and extraction behavior during formulation and purification

Q40. If D (pH 7.4) for a drug is low, what implication for oral absorption might this have?

  • High likelihood of crossing lipid membranes rapidly
  • Potential poor membrane permeability and low absorption
  • Immediate degradation in GI tract
  • No relevance to absorption

Correct Answer: Potential poor membrane permeability and low absorption

Q41. Which technique helps separate and quantify small molecules between phases without complete phase separation?

  • Mass spectrometry on mixed phases without separation
  • Use of radiolabeled tracers and liquid scintillation counting after phase separation
  • Measuring pH only
  • Gas chromatography for non-volatile solutes in aqueous solution

Correct Answer: Use of radiolabeled tracers and liquid scintillation counting after phase separation

Q42. Which statement is true when a molecule forms hydrogen bonds strongly with water?

  • It tends to be more lipophilic
  • It tends to be more hydrophilic and prefers aqueous phase
  • Hydrogen bonding does not affect partitioning
  • It will necessarily precipitate in both phases

Correct Answer: It tends to be more hydrophilic and prefers aqueous phase

Q43. For a monoprotic weak acid with pKa 4 and pH 2 in aqueous phase, the fraction non-ionized is approximately:

  • 1% non-ionized
  • 99% non-ionized
  • 50% non-ionized
  • 0% non-ionized

Correct Answer: 99% non-ionized

Q44. If a drug exhibits different K values when measured between two solvent pairs (octanol/water vs chloroform/water), this illustrates:

  • The universality of K
  • Solvent-specific interactions affecting partitioning
  • That K is independent of solvent choice
  • Measurement error only

Correct Answer: Solvent-specific interactions affecting partitioning

Q45. Which is a correct approach to increase extraction of an acidic drug into organic solvent?

  • Increase pH to fully ionize the acid
  • Decrease pH to convert acid to non-ionized form
  • Add strong base to aqueous phase
  • Decrease organic solvent volume drastically

Correct Answer: Decrease pH to convert acid to non-ionized form

Q46. In the context of distribution law, what does “activity” account for that concentration does not?

  • Only the molecular weight
  • Non-ideal interactions and effective thermodynamic concentration
  • Color and odor of solute
  • Volume of the container

Correct Answer: Non-ideal interactions and effective thermodynamic concentration

Q47. A drug with log P ~ 0 is expected to be:

  • Highly lipophilic
  • Equally distributed between octanol and water
  • Completely insoluble in water
  • Always ionized

Correct Answer: Equally distributed between octanol and water

Q48. Which is TRUE about the use of octanol-water partitioning in drug development?

  • It gives direct measurement of in vivo distribution in all tissues
  • It provides an empirical measure of lipophilicity that correlates with membrane permeability and ADME properties
  • It replaces all biological testing
  • It is only relevant for proteins

Correct Answer: It provides an empirical measure of lipophilicity that correlates with membrane permeability and ADME properties

Q49. If a weak base has pKa 9 and the medium is pH 7, the fraction of unionized base (approx) is:

  • 99% unionized
  • 1% unionized
  • 50% unionized
  • 0% unionized

Correct Answer: 1% unionized

Q50. Why is understanding distribution law important for designing liquid–liquid extractions in pharmaceutical labs?

  • It helps select pH, solvent, and number of extractions to maximize recovery and purity
  • It dictates the color of the final product
  • It is only theoretical with no lab relevance
  • It replaces need for analytical assays

Correct Answer: It helps select pH, solvent, and number of extractions to maximize recovery and purity

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