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
