Unit Operation: Liquid–liquid extraction methods and equilibria MCQs With Answer

Unit Operation: Liquid–liquid extraction methods and equilibria MCQs With Answer

This quiz collection covers core concepts of liquid–liquid extraction tailored for M.Pharm students studying MPC 204T Pharmaceutical Process Chemistry. It reviews fundamental equilibria (distribution/partition coefficient, Nernst distribution law), practical extraction methods (batch, counter-current, mixer-settler, centrifuge contactor), phase behavior (ternary diagrams, tie-lines, binodal curve), and process enhancement techniques (pH adjustment, salting-out, complexation). Questions emphasize solvent selection, stage calculations, extraction factor, and troubleshooting issues such as emulsions and third-phase formation. These MCQs are designed to deepen conceptual understanding and support exam readiness by linking theory to pharmaceutical separation practice.

Q1. What does the distribution (partition) coefficient (KD) represent in liquid–liquid extraction?

• The ratio of solute mass to solvent mass in a single phase
• The ratio of solute concentration in the solvent phase to that in the raffinate phase at equilibrium
• The ratio of solvent volume to feed volume required for extraction
• The volume fraction of solvent recovered after extraction

Correct Answer: The ratio of solute concentration in the solvent phase to that in the raffinate phase at equilibrium

Q2. According to the Nernst distribution law, when is the law strictly applicable?

• When chemical reactions occur between solute and solvent
• When solute exists in both phases without association or dissociation and temperature is constant
• When there is significant mutual solubility of the two liquid phases
• When emulsification prevents phase separation

Correct Answer: When solute exists in both phases without association or dissociation and temperature is constant

Q3. Which parameter describes the effectiveness of extraction in a single stage when solvent and feed volumes are considered?

• Partition selectivity
• Extraction factor (E)
• Mass transfer coefficient
• Interfacial tension

Correct Answer: Extraction factor (E)

Q4. The extraction factor E is defined as KD × (Vs/Va). What does a high value of E indicate?

• Poor extraction requiring many stages
• High per-stage removal of solute into the solvent phase
• High likelihood of emulsion formation
• That the process is limited by diffusion across the interface

Correct Answer: High per-stage removal of solute into the solvent phase

Q5. In acid-base extraction of a weak acid, what is the main reason pH adjustment increases extraction into the aqueous phase?

• Changing solvent density to favor mixing
• Converting the acid into its ionized (water-soluble) form
• Increasing mutual solubility of the two liquids
• Reducing the distribution coefficient KD

Correct Answer: Converting the acid into its ionized (water-soluble) form

Q6. Which of the following is NOT a desirable property for an organic solvent used in pharmaceutical liquid–liquid extraction?

• Low solubility in the aqueous phase
• High boiling point to avoid losses during processing
• High selectivity (high KD for target solute)
• Low toxicity and easy recoverability

Correct Answer: High boiling point to avoid losses during processing

Q7. What is a tie-line in a ternary phase diagram for liquid–liquid extraction?

• The boundary separating single-phase and two-phase regions
• A line connecting compositions of coexisting equilibrium phases
• A line showing the boiling points of components
• The path of a batch extraction process

Correct Answer: A line connecting compositions of coexisting equilibrium phases

Q8. The binodal curve in a ternary diagram represents:

• All possible feed compositions that require heating
• The locus of compositions at which two liquid phases first form (limit of miscibility)
• The line of maximum solute solubility in the solvent
• The region where chemical reactions are favored

Correct Answer: The locus of compositions at which two liquid phases first form (limit of miscibility)

Q9. Which operational configuration typically yields the highest extraction efficiency for a given solvent flowrate and distribution coefficient?

• Single-stage batch extraction
• Cross-current multiple equal-stage extraction
• Counter-current multi-stage extraction
• Sequential single-drop extractions

Correct Answer: Counter-current multi-stage extraction

Q10. What is the primary advantage of using a mixer-settler unit in pharmaceutical liquid–liquid extraction?

• It eliminates the need for solvent selection
• It provides separate stages for intimate mixing and gentle phase separation
• It guarantees single-drop equilibrium attainment
• It always prevents emulsion formation

Correct Answer: It provides separate stages for intimate mixing and gentle phase separation

Q11. Salting-out improves extraction by:

• Increasing mutual solubility of organic and aqueous phases
• Decreasing the solubility of solute in the aqueous phase to favor transfer into the organic phase
• Forming micelles that solubilize the solute
• Lowering the interfacial tension to speed settling

Correct Answer: Decreasing the solubility of solute in the aqueous phase to favor transfer into the organic phase

Q12. Which mechanism primarily controls mass transfer in a well-mixed liquid–liquid extraction stage with small drop sizes?

• Convective transport across large eddies only
• Molecular diffusion across the liquid–liquid interface and within the drops
• Bulk chemical reaction in the continuous phase exclusively
• Vapor-phase transport between liquids

Correct Answer: Molecular diffusion across the liquid–liquid interface and within the drops

Q13. Third-phase formation during solvent extraction is most often caused by:

• Too low solute concentration in the feed
• Formation of a viscous, solvent-rich middle layer due to excessive complex or surfactant concentration
• Excessive interfacial tension leading to rapid coalescence
• Using highly volatile solvents at low temperature

Correct Answer: Formation of a viscous, solvent-rich middle layer due to excessive complex or surfactant concentration

Q14. Selectivity (α) between two solutes A and B in an extraction system is defined as:

• KD(A) / KD(B)
• KD(A) × KD(B)
• KD(A) – KD(B)
• The reciprocal of the extraction factor

Correct Answer: KD(A) / KD(B)

Q15. Which device uses centrifugal force to rapidly separate phases in liquid–liquid extraction and is suitable for emulsifying or slow-settling systems?

• Mixer-settler
• Rotary evaporator
• Centrifugal extractor (centrifuge contactor)
• Falling-film column

Correct Answer: Centrifugal extractor (centrifuge contactor)

Q16. In multistage counter-current extraction, increasing solvent-to-feed ratio while maintaining KD constant generally results in:

• More solvent loss to the raffinate
• Lower overall extraction of solute
• Higher purity of extract and higher overall solute recovery
• Immediate increase in emulsion formation

Correct Answer: Higher purity of extract and higher overall solute recovery

Q17. Which practice can be used to break stable emulsions formed during extraction?

• Reduce temperature to freeze the phases slightly
• Add salts or demulsifying agents and apply gentle heating or centrifugation
• Increase stirring speed indefinitely
• Replace solvent with one that is completely miscible with the aqueous phase

Correct Answer: Add salts or demulsifying agents and apply gentle heating or centrifugation

Q18. For an extractive crystallization approach following solvent extraction, which solvent property is most important to enable easy solvent removal before drying?

• High viscosity at room temperature
• Low boiling point and ease of recovery by distillation
• High mutual solubility with water
• Strong complexation with the solute

Correct Answer: Low boiling point and ease of recovery by distillation

Q19. In a single equal-stage extraction of solute from aqueous feed A with solvent S, if KD is very large (>100) and phase volumes are similar, the expected result is:

• Most solute remains in the aqueous raffinate
• Nearly complete transfer of solute to the solvent phase
• Equilibrium strongly favors the aqueous phase
• No effect because KD must be <1 to extract

Correct Answer: Nearly complete transfer of solute to the solvent phase

Q20. Which equilibrium tool helps determine how much of each phase will be present and their compositions after mixing a given feed and solvent?

• Mass transfer coefficient chart
• Ternary phase diagram with lever rule and tie-lines
• Gas–liquid equilibrium table
• Hildebrand solubility parameter table only

Correct Answer: Ternary phase diagram with lever rule and tie-lines

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