Dipole moment – determination and applications MCQs With Answer

Dipole moment – determination and applications MCQs With Answer

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Understanding dipole moment — its determination and applications — is essential for B.Pharm students. This topic explains dipole moment as a molecular vector, units (Debye), experimental determination methods (dielectric measurements, refractometry, microwave and spectroscopic techniques), theoretical estimation (bond dipole additivity, quantum calculations), and pharmaceutical applications such as solubility prediction, partitioning, drug–receptor interactions, formulation stability and SAR analysis. The MCQs cover theory, experimental techniques, calculations and pharmaceutical case studies to build exam-ready competence. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the dipole moment of a molecule?

  • A quantitative measure of the separation of positive and negative charges in a molecule
  • The total number of polar bonds in a molecule
  • The ability of a molecule to rotate in an electric field
  • The molecular mass adjusted for polarity

Correct Answer: A quantitative measure of the separation of positive and negative charges in a molecule

Q2. Which unit is commonly used to express molecular dipole moments?

  • Volt
  • Debye
  • Newton
  • Farad

Correct Answer: Debye

Q3. Which of the following is the approximate value of 1 Debye in SI units?

  • 3.33564 × 10^-30 C·m
  • 1.602 × 10^-19 C·m
  • 6.022 × 10^23 C·m
  • 9.109 × 10^-31 C·m

Correct Answer: 3.33564 × 10^-30 C·m

Q4. Dipole moment is a vector quantity. What does this imply?

  • It has magnitude only
  • It has both magnitude and direction
  • It changes sign with temperature
  • It depends solely on molecular mass

Correct Answer: It has both magnitude and direction

Q5. For which molecule is the net dipole moment zero despite having polar bonds?

  • H2O
  • CO2
  • HF
  • NH3

Correct Answer: CO2

Q6. Which experimental method is commonly used to determine dipole moment in liquids?

  • Dielectric constant measurement
  • Mass spectrometry
  • Polarimetry
  • Gel electrophoresis

Correct Answer: Dielectric constant measurement

Q7. The Debye equation relates dipole moment to which measurable properties?

  • Dielectric constant, refractive index and temperature
  • Boiling point and melting point
  • Optical rotation and viscosity
  • Molecular weight and pKa

Correct Answer: Dielectric constant, refractive index and temperature

Q8. How does molecular symmetry affect dipole moment?

  • High symmetry always increases dipole moment
  • Perfectly symmetric charge distribution can lead to zero dipole moment
  • Symmetry has no effect on dipole moment
  • Symmetry only affects induced dipoles, not permanent ones

Correct Answer: Perfectly symmetric charge distribution can lead to zero dipole moment

Q9. Which statement best describes a permanent dipole?

  • A dipole induced only when an external field is applied
  • A permanent separation of charge due to polar bonds or molecular geometry
  • A temporary fluctuation in the electron cloud only present at high temperature
  • A dipole observed only in ionic crystals

Correct Answer: A permanent separation of charge due to polar bonds or molecular geometry

Q10. Which of the following influences the dipole moment of a drug molecule significantly?

  • The number of carbon atoms only
  • Presence and orientation of polar functional groups
  • Atomic weight of nonpolar substituents only
  • Crystal lattice structure in solid state only

Correct Answer: Presence and orientation of polar functional groups

Q11. Why is dipole moment important in predicting solubility of drugs?

  • Higher dipole moment always yields lower aqueous solubility
  • Dipole moment indicates polarity which affects interaction with polar solvents like water
  • Dipole moment is only relevant for gaseous compounds
  • Solubility is unrelated to dipole moment

Correct Answer: Dipole moment indicates polarity which affects interaction with polar solvents like water

Q12. In medicinal chemistry, dipole moment is used as a descriptor in QSAR primarily because:

  • It quantifies molecular polarity affecting biological interactions
  • It measures molecular weight accurately
  • It predicts melting point with high accuracy
  • It directly gives the binding constant to a receptor

Correct Answer: It quantifies molecular polarity affecting biological interactions

Q13. Which spectroscopic technique can provide information related to molecular dipole transitions?

  • Infrared (IR) spectroscopy
  • Atomic absorption spectroscopy
  • Flame photometry
  • Polarography

Correct Answer: Infrared (IR) spectroscopy

Q14. How does conformation affect dipole moment?

  • Conformation cannot change dipole moment for the same connectivity
  • Differing conformations can change vector sum of bond dipoles and thus dipole moment
  • Only bond lengths affect dipole moment, not conformation
  • Conformation only affects induced dipoles

Correct Answer: Differing conformations can change vector sum of bond dipoles and thus dipole moment

Q15. The dipole moment of a molecule can often be estimated by:

  • Simple addition of atomic masses
  • Vector addition of individual bond dipoles
  • Counting number of hydrogen atoms only
  • Measuring optical rotation

Correct Answer: Vector addition of individual bond dipoles

Q16. Which factor is NOT directly used in the dielectric method for dipole determination?

  • Dielectric constant of the medium
  • Refractive index
  • Solution concentration
  • Protein binding constant

Correct Answer: Protein binding constant

Q17. What does the Kirkwood g-factor account for in dipole measurements?

  • Quantum corrections to dipole moment
  • Orientational correlation between neighboring dipoles in a liquid
  • The mass of the solvent molecules
  • Magnetic interactions between molecules

Correct Answer: Orientational correlation between neighboring dipoles in a liquid

Q18. Which type of intermolecular force is directly related to permanent dipole moments?

  • London dispersion forces
  • Dipole–dipole interactions
  • Metallic bonding
  • Covalent bonding

Correct Answer: Dipole–dipole interactions

Q19. How does increased molecular dipole moment usually affect boiling point?

  • Increases boiling point due to stronger dipole–dipole attractions
  • Decreases boiling point due to faster evaporation
  • Has no effect on boiling point
  • Always causes decomposition before boiling

Correct Answer: Increases boiling point due to stronger dipole–dipole attractions

Q20. Which of the following describes an induced dipole?

  • A permanent separation of charge in a molecule
  • A dipole created in a nonpolar molecule by an external electric field or nearby dipole
  • A dipole that exists only in the gas phase
  • A dipole measured in Debye units only

Correct Answer: A dipole created in a nonpolar molecule by an external electric field or nearby dipole

Q21. Which molecular property correlates closely with dielectric constant of a liquid?

  • Average molecular dipole moment and polarizability
  • Number of stereocenters only
  • Isotopic composition
  • Solid-state crystal habit

Correct Answer: Average molecular dipole moment and polarizability

Q22. Microwave spectroscopy helps determine dipole moments because:

  • Rotational transitions depend on the permanent dipole moment
  • Microwave photons break covalent bonds revealing dipoles
  • Microwave only measures vibrational energies unrelated to dipoles
  • It measures refractive index directly

Correct Answer: Rotational transitions depend on the permanent dipole moment

Q23. Which molecule has a higher dipole moment: HCl or H2?

  • H2
  • HCl
  • Both have equal dipole moments
  • Neither has a dipole moment

Correct Answer: HCl

Q24. The direction of dipole moment vector is conventionally from:

  • Negative center to positive center
  • Positive center to negative center
  • Higher atomic number to lower atomic number
  • Clockwise around molecule

Correct Answer: Positive center to negative center

Q25. Resonance can affect dipole moment by:

  • Always increasing dipole moment
  • Delocalizing charge and therefore altering the net dipole
  • Only changing mass but not dipole
  • Converting permanent dipoles into induced dipoles

Correct Answer: Delocalizing charge and therefore altering the net dipole

Q26. Which of these is an application of dipole moment in pharmaceutical sciences?

  • Predicting drug solubility and membrane permeability
  • Measuring tablet hardness directly
  • Determining amino acid sequence of a protein
  • Estimating shelf life by color change

Correct Answer: Predicting drug solubility and membrane permeability

Q27. How does an increase in dipole moment typically affect partition coefficient (log P) between octanol and water?

  • Always increases log P (more lipophilic)
  • May decrease log P by increasing affinity for water (more hydrophilic)
  • Has no effect on log P
  • Causes unpredictable change unrelated to polarity

Correct Answer: May decrease log P by increasing affinity for water (more hydrophilic)

Q28. Which technique measures rotational relaxation times related to dipole reorientation?

  • NMR relaxation and dielectric relaxation spectroscopy
  • Paper chromatography
  • Mass spectrometry
  • Potentiometry

Correct Answer: NMR relaxation and dielectric relaxation spectroscopy

Q29. London dispersion forces differ from dipole–dipole interactions because they:

  • Arise from instantaneous induced dipoles and exist in all molecules
  • Require permanent dipoles to operate
  • Are present only in ionic compounds
  • Are negligible compared to covalent bonds

Correct Answer: Arise from instantaneous induced dipoles and exist in all molecules

Q30. Dipole moment contributes to IR absorption intensity because:

  • IR transitions require a change in molecular dipole during vibration
  • IR depends only on mass, not dipole
  • IR intensity is inversely proportional to dipole moment
  • Only UV-visible transitions depend on dipole changes

Correct Answer: IR transitions require a change in molecular dipole during vibration

Q31. In gas-phase dipole measurements compared to liquid-phase, which statement is true?

  • Gas-phase measurements are influenced more by intermolecular orientation
  • Gas-phase values represent isolated molecular dipole without solvent interactions
  • Liquid-phase always gives higher dipole due to vacuum conditions
  • Both phases yield identical dipole moments for all molecules

Correct Answer: Gas-phase values represent isolated molecular dipole without solvent interactions

Q32. Which functional group addition is most likely to increase a drug’s dipole moment?

  • Methyl (-CH3)
  • Nitro (-NO2)
  • Phenyl (-C6H5)
  • Alkyl chain extension

Correct Answer: Nitro (-NO2)

Q33. How does dipole moment influence drug–receptor binding?

  • Polar interactions and orientation can enhance complementary binding to charged or polar receptor sites
  • Dipole moment only decreases binding affinity always
  • Dipole moment only matters for covalent inhibitors
  • Receptor binding is unaffected by molecular polarity

Correct Answer: Polar interactions and orientation can enhance complementary binding to charged or polar receptor sites

Q34. Which of the following best distinguishes polarizability from dipole moment?

  • Polarizability is the ease of inducing a dipole; dipole moment is a permanent charge separation
  • Polarizability measures permanent charge separation only
  • Dipole moment measures mass distribution; polarizability is a thermal property
  • They are identical physical quantities

Correct Answer: Polarizability is the ease of inducing a dipole; dipole moment is a permanent charge separation

Q35. Which classic example demonstrates that bond polarity does not always yield a net dipole?

  • Water (H2O)
  • Carbon dioxide (CO2)
  • Hydrogen chloride (HCl)
  • Formaldehyde (H2CO)

Correct Answer: Carbon dioxide (CO2)

Q36. Solvatochromism (shift in UV-Vis absorption with solvent polarity) is influenced by:

  • Molecular dipole moment changes between ground and excited states
  • Only molecular weight changes
  • Color impurities only
  • pH but never dipole moment

Correct Answer: Molecular dipole moment changes between ground and excited states

Q37. A drug with a very high dipole moment is likely to have which property?

  • High lipid membrane permeability
  • Increased aqueous solubility and potentially lower passive permeability through lipid bilayers
  • Complete insolubility in water
  • No interaction with polar proteins

Correct Answer: Increased aqueous solubility and potentially lower passive permeability through lipid bilayers

Q38. Which effect can reduce the measured dipole moment of a conjugated system?

  • Charge delocalization through resonance
  • Introduction of a highly polar substituent aligned with bond dipoles
  • Breaking all pi bonds
  • Adding an ionic counterion

Correct Answer: Charge delocalization through resonance

Q39. Dipole–induced dipole interactions are also called:

  • Keesom forces
  • Debye forces
  • London dispersion forces
  • Hydrogen bonds

Correct Answer: Debye forces

Q40. Which is the approximate dipole moment of a water molecule?

  • 0.1 D
  • 1.85 D
  • 10 D
  • 100 D

Correct Answer: 1.85 D

Q41. For HCl, the dipole arrow (conventionally pointing from positive to negative) points toward:

  • Hydrogen
  • Chlorine
  • Midpoint between H and Cl
  • Opposite to chlorine

Correct Answer: Chlorine

Q42. In vector addition of bond dipoles, what determines the net dipole?

  • Magnitude and direction of each bond dipole and molecular geometry
  • Only the largest bond dipole magnitude
  • Sum of atomic numbers
  • Only the number of polar bonds

Correct Answer: Magnitude and direction of each bond dipole and molecular geometry

Q43. Which property is most directly increased by adding ionic groups rather than neutral polar groups?

  • Permanent dipole moment always
  • Water solubility dramatically due to ionization
  • Lipid solubility always
  • Refractive index only

Correct Answer: Water solubility dramatically due to ionization

Q44. Which statement about temperature effect on measured dipole moment in dielectric experiments is correct?

  • Temperature has no effect on dielectric measurements
  • Thermal motion affects orientation and can change measured effective dipole-related parameters
  • Raising temperature always increases permanent dipole moment
  • Only pressure affects dielectric constant, not temperature

Correct Answer: Thermal motion affects orientation and can change measured effective dipole-related parameters

Q45. Which approach gives the most accurate theoretical dipole moment for a complex drug molecule?

  • Empirical additivity only
  • High-level quantum chemical calculations (ab initio/DFT)
  • Estimating from molecular weight
  • Counting heteroatoms without geometry

Correct Answer: High-level quantum chemical calculations (ab initio/DFT)

Q46. London dispersion forces are strongest in molecules that are:

  • Highly polar only
  • Large and highly polarizable
  • Small and rigid
  • Always ionic

Correct Answer: Large and highly polarizable

Q47. Which process best demonstrates the practical relevance of dipole moment in formulation?

  • Predicting miscibility of solvents and excipients based on polarity
  • Measuring tablet tensile strength
  • Predicting color changes on exposure to light
  • Establishing microbial limits

Correct Answer: Predicting miscibility of solvents and excipients based on polarity

Q48. Instantaneous dipoles that give rise to dispersion forces are due to:

  • Permanent ionic charges
  • Fluctuations in the electron cloud distribution
  • Alignment of permanent dipoles only
  • Thermal decomposition

Correct Answer: Fluctuations in the electron cloud distribution

Q49. Which change will most likely decrease the dipole moment of a molecule?

  • Adding a strongly electron-withdrawing group aligned with existing bond dipoles
  • Introducing resonance that delocalizes charge opposite to bond dipoles
  • Converting a nonpolar substituent to a polar one with same orientation
  • Changing conformation to align polar groups in the same direction

Correct Answer: Introducing resonance that delocalizes charge opposite to bond dipoles

Q50. The primary physical significance of molecular dipole moment in pharmaceutical context is:

  • It directly gives the pKa of the molecule
  • It helps predict intermolecular interactions, solvation and partitioning relevant to drug action
  • It measures the number of stereocenters
  • It quantifies crystalline hardness

Correct Answer: It helps predict intermolecular interactions, solvation and partitioning relevant to drug action

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