Classification of complexation MCQs With Answer

Classification of complexation MCQs With Answer

Understanding the classification of complexation is essential for B. Pharm students studying drug formulation, stability, and delivery. This concise guide covers key keywords such as complexation chemistry, pharmaceutical complexation, inclusion complexes, chelation, cyclodextrin complexes, metal complexes, stability constants and host–guest interactions. It emphasizes types of complexes, mechanisms, stoichiometry (1:1, 1:2, 2:1), analytical methods (Job’s plot, Benesi–Hildebrand, spectrophotometry, NMR), and formulation applications like solubility enhancement and controlled release. Simple, practical and exam-focused, this resource prepares students to recognize, classify and evaluate complexation phenomena in pharmaceutics. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What does the term “complexation” primarily refer to in pharmaceutical chemistry?

• Formation of a covalent bond between two drug molecules
• Association between a central atom or molecule and one or more ligands via coordinate or non-covalent interactions
• Polymerization of monomers under heat
• Oxidation of drug molecules in presence of oxygen

Correct Answer: Association between a central atom or molecule and one or more ligands via coordinate or non-covalent interactions

Q2. Which of the following is NOT a common classification criterion for complexes?

• By nature of bonding (coordinate, ionic, hydrogen-bonded)
• By stoichiometry (1:1, 1:2, 2:1)
• By melting point of the complex
• By application (inclusion, chelation, ion-pairing)

Correct Answer: By melting point of the complex

Q3. Inclusion complexes are best exemplified by which host molecule commonly used in pharmaceutics?

• EDTA
• Cyclodextrin
• Crown ether
• Amoxicillin

Correct Answer: Cyclodextrin

Q4. Chelation primarily involves which type of bonding interaction?

• Multiple coordinate bonds from a multidentate ligand to a central metal ion
• Ionic attraction between opposite charges
• Hydrophobic interactions only
• Van der Waals forces exclusively

Correct Answer: Multiple coordinate bonds from a multidentate ligand to a central metal ion

Q5. The formation (stability) constant Kf for a 1:1 complex ML is defined as:

• Kf = [M][L] / [ML]
• Kf = [ML] / ([M][L])
• Kf = [M] + [L] + [ML]
• Kf = [ML] / ([M] + [L])

Correct Answer: Kf = [ML] / ([M][L])

Q6. Job’s method (method of continuous variation) is used to determine:

• Complex stoichiometry
• Absolute concentration of a reagent
• Boiling point of a solution
• Molecular weight of polymers

Correct Answer: Complex stoichiometry

Q7. Which analytical technique is commonly used to determine formation constants spectrophotometrically?

• Benesi–Hildebrand method
• Thin layer chromatography
• Gel permeation chromatography
• Karl Fischer titration

Correct Answer: Benesi–Hildebrand method

Q8. A 2:1 host–guest complex denotes which stoichiometry?

• 2 guests : 1 host
• 2 hosts : 1 guest
• 1 host : 1 guest
• 3 hosts : 1 guest

Correct Answer: 2 hosts : 1 guest

Q9. Which factor typically increases complex stability for metal–ligand complexes?

• Higher ionic strength
• Ligand with higher denticity
• Lower ligand concentration
• Presence of strong competing solvent interactions

Correct Answer: Ligand with higher denticity

Q10. Crown ethers are primarily used to complex which type of species?

• Neutral hydrophobic drugs only
• Alkali metal cations (e.g., K+, Na+)
• Large proteins
• Water molecules

Correct Answer: Alkali metal cations (e.g., K+, Na+)

Q11. Which donor atoms are most common in pharmaceutical chelating agents?

• O, N, S
• He, Ne, Ar
• Au, Pt, Hg
• F, Cl, Br

Correct Answer: O, N, S

Q12. Cyclodextrin complexation enhances drug properties mainly by:

• Decreasing molecular weight
• Forming covalent bonds with drugs
• Enhancing solubility and stability via inclusion of hydrophobic drug moieties
• Oxidizing the drug molecule

Correct Answer: Enhancing solubility and stability via inclusion of hydrophobic drug moieties

Q13. Ion-pair complexation is most useful for improving:

• Drug crystallinity
• Transmembrane permeation and extraction of ionic drugs
• Polymer crosslinking
• Vaccine antigenicity

Correct Answer: Transmembrane permeation and extraction of ionic drugs

Q14. Which statement correctly distinguishes co-crystals from inclusion complexes?

• Co-crystals involve a guest enclosed in a host cavity; inclusion complexes are stoichiometric crystals
• Co-crystals are crystalline solids of two or more components in a definite stoichiometric ratio held by non-covalent interactions; inclusion complexes involve a guest trapped within a host cavity
• They are identical terms and interchangeable
• Co-crystals always involve metal ions

Correct Answer: Co-crystals are crystalline solids of two or more components in a definite stoichiometric ratio held by non-covalent interactions; inclusion complexes involve a guest trapped within a host cavity

Q15. Which experimental technique provides direct thermodynamic parameters (ΔH, ΔS) for complex formation?

• Isothermal titration calorimetry (ITC)
• Mass spectrometry
• Polarimetry
• Paper chromatography

Correct Answer: Isothermal titration calorimetry (ITC)

Q16. In complexation, “denticity” refers to:

• The number of donor atoms in a ligand that bind to a central atom
• The temperature at which a complex melts
• The electrical conductivity of a complex
• The rate of complex formation

Correct Answer: The number of donor atoms in a ligand that bind to a central atom

Q17. EDTA is widely used in pharmacy as a:

• Surfactant
• Chelating agent to sequester metal ions
• Primary preservative
• Solvent for parenteral drugs

Correct Answer: Chelating agent to sequester metal ions

Q18. A high formation constant (Kf) indicates:

• Very weak binding between components
• Strong or thermodynamically favorable complex formation
• That the complex is always irreversible
• That the complex will be insoluble

Correct Answer: Strong or thermodynamically favorable complex formation

Q19. Which factor tends to decrease the stability of hydrogen-bonded complexes?

• Lower temperature
• Presence of competing hydrogen-bonding solvent (e.g., water)
• Removing water from the system
• Using aprotic solvents

Correct Answer: Presence of competing hydrogen-bonding solvent (e.g., water)

Q20. The Benesi–Hildebrand plot is linear for which stoichiometry under appropriate assumptions?

• 1:1 complex
• 2:3 complex
• 2:2 complex only
• No complex can yield linearity

Correct Answer: 1:1 complex

Q21. Host–guest chemistry often relies on which interactions between host and guest?

• Covalent bonding only
• Non-covalent interactions such as hydrophobic, van der Waals, hydrogen bonding and electrostatic forces
• Radioactive decay processes
• Polymerization reactions

Correct Answer: Non-covalent interactions such as hydrophobic, van der Waals, hydrogen bonding and electrostatic forces

Q22. Which of the following is a pharmaceutical application of complexation?

• Masking taste of bitter drugs via inclusion complexes
• Increasing drug toxicity without benefit
• Preventing drug–protein interactions indefinitely
• Eliminating the need for sterilization

Correct Answer: Masking taste of bitter drugs via inclusion complexes

Q23. In complexometric titration, which indicator changes color upon binding of a metal ion?

• EDTA
• Eriochrome Black T
• Benzene
• Cyclodextrin

Correct Answer: Eriochrome Black T

Q24. Which term best describes a ligand that binds through two donor atoms?

• Monodentate
• Bidentate
• Polystyrene
• Neutral ligand

Correct Answer: Bidentate

Q25. Spectroscopic shifts in NMR upon complexation typically indicate:

• No change in electronic environment
• Change in electronic environment around nuclei due to binding
• Only formation of covalent bonds
• That the complex precipitated

Correct Answer: Change in electronic environment around nuclei due to binding

Q26. Which complex type is most reversible and useful for controlled drug release?

• Irreversible covalent adducts
• Weak host–guest inclusion complexes
• Metal–carbonyl complexes
• Permanently crosslinked polymers

Correct Answer: Weak host–guest inclusion complexes

Q27. Which property of a drug is commonly improved by forming an inclusion complex with cyclodextrin?

• Thermal conductivity
• Aqueous solubility and chemical stability
• Radioactivity
• Intrinsic pKa

Correct Answer: Aqueous solubility and chemical stability

Q28. Metal–ligand coordination number refers to:

• Number of ligands attached to a ligand
• Number of coordinate bonds formed by the central atom
• Number of electrons in the ligand
• Molecular weight of the metal

Correct Answer: Number of coordinate bonds formed by the central atom

Q29. A chelate effect describes:

• Decreased stability of polydentate ligands vs monodentate ligands
• Enhanced stability of complexes formed by multidentate ligands compared to equivalent monodentate ligands
• Polymer degradation in presence of metal ions
• Electrostatic repulsion between ligands

Correct Answer: Enhanced stability of complexes formed by multidentate ligands compared to equivalent monodentate ligands

Q30. Which method can determine stoichiometry and formation constant simultaneously from spectrophotometric data?

• Job’s method alone
• Non-linear curve-fitting of absorbance vs concentration data using a binding model
• Paper chromatography
• pH titration without indicators

Correct Answer: Non-linear curve-fitting of absorbance vs concentration data using a binding model

Q31. Which interaction is least likely to form a stable drug–host inclusion complex?

• Hydrophobic interactions between drug aromatic rings and hydrophobic cavity
• Strong covalent bond formation within the cavity
• Van der Waals complementarity of host cavity and guest
• Hydrogen bonding at the cavity rim

Correct Answer: Strong covalent bond formation within the cavity

Q32. Which is a typical experimental sign that a solid-state complex has formed between drug and cyclodextrin?

• Unchanged DSC thermogram compared to physical mixture
• Disappearance or shift of drug melting endotherm in DSC
• Increase in crystalline peaks of the drug in XRD
• Complete insolubility in all solvents

Correct Answer: Disappearance or shift of drug melting endotherm in DSC

Q33. Which pharmaceutical issue can chelation to metal ions help to prevent?

• Metal-catalyzed degradation of drug molecules
• Increased drug volatility
• Formation of covalent adducts with excipients
• Enhanced absorption across all membranes

Correct Answer: Metal-catalyzed degradation of drug molecules

Q34. The term “host” in host–guest chemistry refers to:

• The guest molecule entering the cavity
• The larger molecule or structure providing a binding cavity
• Only inorganic salts
• A solvent molecule

Correct Answer: The larger molecule or structure providing a binding cavity

Q35. Which of the following is a common method to characterize complexes in solution?

• UV–Visible spectrophotometry and NMR
• Melting point of pure solvent
• Paper chromatographic Rf of water
• Density of solid excipients

Correct Answer: UV–Visible spectrophotometry and NMR

Q36. Which statement about ion-pair complexes is correct?

• They form between two neutral molecules
• They result from electrostatic attraction between oppositely charged ions
• They always decrease membrane permeability
• They are synonymous with covalent complexes

Correct Answer: They result from electrostatic attraction between oppositely charged ions

Q37. Which property of a ligand increases its ability to solubilize lipophilic drugs via complexation?

• High hydrophilicity of the entire ligand with a hydrophobic cavity (e.g., cyclodextrin)
• Extremely high volatility
• High thermal decomposition rate
• Complete insolubility in water

Correct Answer: High hydrophilicity of the entire ligand with a hydrophobic cavity (e.g., cyclodextrin)

Q38. Which of these is an effect of complexation on drug pharmacokinetics?

• Unchanged absorption regardless of complex stability
• Altered bioavailability due to changes in solubility and permeability
• Immediate permanent inactivation of the drug
• Complete elimination of metabolism

Correct Answer: Altered bioavailability due to changes in solubility and permeability

Q39. A ligand that donates a lone pair to form a coordinate bond is called:

• An electrophile
• A Lewis base
• A radical
• An oxidizing agent

Correct Answer: A Lewis base

Q40. Which complexation type is particularly useful for removing toxic heavy metals from the body?

• Ion-pair complexation with lipids
• Chelation therapy using chelating agents like EDTA
• Inclusion complexes with cyclodextrin
• Hydrophobic aggregation

Correct Answer: Chelation therapy using chelating agents like EDTA

Q41. Which parameter is most directly measured in a spectrophotometric titration to evaluate complex formation?

• Absorbance at a specific wavelength
• Viscosity of solution
• Boiling point elevation
• pH only

Correct Answer: Absorbance at a specific wavelength

Q42. Which complex is likely to enhance transdermal delivery of an ionized drug?

• Strongly hydrophilic polymer complex
• Ion-pair complex with a lipophilic counter-ion
• Inclusion complex that makes the drug more hydrophilic
• Complex that increases molecular size drastically

Correct Answer: Ion-pair complex with a lipophilic counter-ion

Q43. Which descriptor best classifies complexes formed by hydrogen bonding and dipole interactions without metal centers?

• Coordination complexes
• Supramolecular complexes (non-covalent host–guest)
• Organometallic complexes
• Isotopic complexes

Correct Answer: Supramolecular complexes (non-covalent host–guest)

Q44. In a solution where a drug forms a 1:1 complex with a host, increasing host concentration beyond a certain point will:

• Always precipitate the drug
• Shift equilibrium to favor more complex formation until saturation is reached
• Decrease complex formation indefinitely
• Destroy the drug chemically

Correct Answer: Shift equilibrium to favor more complex formation until saturation is reached

Q45. Which technique can differentiate between physical mixture and true complex in solid state?

• X-ray diffraction (XRD) and differential scanning calorimetry (DSC)
• Simple visual inspection only
• Measuring color under fluorescent light only
• pH measurement of the solid

Correct Answer: X-ray diffraction (XRD) and differential scanning calorimetry (DSC)

Q46. What is the main pharmaceutical advantage of reversible complexation?

• Permanent chemical inactivation of drug
• Controlled release and reversible masking of drug properties
• Irreversible binding to plasma proteins
• Complete elimination from formulation needs

Correct Answer: Controlled release and reversible masking of drug properties

Q47. Which of the following best describes a coordination complex?

• An assembly where ligands donate electron pairs to a central metal ion forming coordinate bonds
• A solution of unbound ions only
• A polymer network crosslinked by covalent bonds exclusively
• Molecules held together only by hydrophobic forces

Correct Answer: An assembly where ligands donate electron pairs to a central metal ion forming coordinate bonds

Q48. Which factor is critical when designing a cyclodextrin complex for a given drug?

• Size complementarity between drug moiety and cyclodextrin cavity
• Ability of drug to form covalent bonds with cyclodextrin
• Drug volatility at room temperature
• Drug radioactivity

Correct Answer: Size complementarity between drug moiety and cyclodextrin cavity

Q49. Which is a drawback of excessively high stability constants in drug–host complexes for oral delivery?

• Faster release in biological fluids
• Insufficient drug release at absorption site due to too strong binding
• Improved taste masking always
• Enhanced enzymatic degradation

Correct Answer: Insufficient drug release at absorption site due to too strong binding

Q50. In classifying complexes by reversibility, which description fits most inclusion complexes used in formulations?

• Irreversible covalent adducts
• Reversible, non-covalent host–guest assemblies
• Radioactive decay products
• Permanent ionic precipitates

Correct Answer: Reversible, non-covalent host–guest assemblies

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