Introduction: Optical rotation is a key concept in pharmaceutical analysis that measures how chiral molecules rotate plane-polarized light. B.Pharm students must understand polarimetry, specific rotation, and factors affecting optical activity—wavelength (commonly sodium D line, 589 nm), temperature, concentration, and path length. Mastery of optical rotation aids in determining enantiomeric purity, stereochemistry, and quality control of drugs (sugars, alkaloids, amino acids). This SEO-friendly guide covers determination methods, instrumental principles, calculations, and pharmaceutical applications to strengthen lab competency and theoretical knowledge. ‘Now let’s test your knowledge with 50 MCQs on this topic.’
Q1. What does optical rotation measure in a chiral compound?
- The amount of light absorbed
- The rotation of plane-polarized light
- The refractive index change
- The fluorescence emitted
Correct Answer: The rotation of plane-polarized light
Q2. Which instrument is primarily used to measure optical rotation?
- Spectrophotometer
- Polarimeter
- Chromatograph
- pH meter
Correct Answer: Polarimeter
Q3. The specific rotation [α] is defined as:
- The observed rotation divided by the product of path length and concentration
- The observed rotation multiplied by concentration
- The observed rotation times path length
- The square root of observed rotation
Correct Answer: The observed rotation divided by the product of path length and concentration
Q4. In the specific rotation formula [α] = α / (l·c), what are the usual units for l and c?
- l in cm and c in mol/L
- l in dm and c in g/mL
- l in m and c in g/L
- l in mm and c in mg/mL
Correct Answer: l in dm and c in g/mL
Q5. A compound that rotates plane-polarized light clockwise when viewed toward the light source is termed:
- Levorotatory
- Dextrorotatory
- Achromatic
- Racemate
Correct Answer: Dextrorotatory
Q6. Which wavelength is commonly used in polarimetry for measuring optical rotation?
- UV 254 nm
- Green 532 nm
- Sodium D line 589 nm
- IR 1064 nm
Correct Answer: Sodium D line 589 nm
Q7. What effect does increasing temperature generally have on specific rotation?
- No effect at all temperatures
- May change specific rotation due to conformational changes
- Always makes rotation more positive
- Always makes rotation more negative
Correct Answer: May change specific rotation due to conformational changes
Q8. Optical purity (percent) can be calculated from:
- The ratio of observed rotation to rotation of racemate
- The ratio of observed rotation to rotation of pure enantiomer times 100
- Concentration divided by path length
- Square of specific rotation
Correct Answer: The ratio of observed rotation to rotation of pure enantiomer times 100
Q9. What is a racemic mixture?
- A mixture containing only one enantiomer
- A 50:50 mixture of enantiomers with zero net rotation
- A mixture of diastereomers
- A polymeric mixture
Correct Answer: A 50:50 mixture of enantiomers with zero net rotation
Q10. Circular dichroism (CD) differs from optical rotation because CD measures:
- Difference in absorption of left and right circularly polarized light
- Change in refractive index
- Intensity of transmitted light
- Scattering of light
Correct Answer: Difference in absorption of left and right circularly polarized light
Q11. In polarimetric titration, what is typically monitored?
- pH change
- Change in optical rotation as reaction proceeds
- Conductivity change
- Color change only
Correct Answer: Change in optical rotation as reaction proceeds
Q12. Which of the following phenomena explains optical rotation at a molecular level?
- Chromatographic separation
- Differential interaction with plane-polarized light due to molecular asymmetry
- Ionization in solution
- Thermal expansion
Correct Answer: Differential interaction with plane-polarized light due to molecular asymmetry
Q13. A solution of 1.0 g/mL concentration in a 1.0 dm cell produces an observed rotation of +10°. What is the specific rotation?
- +1°
- +10°
- +100°
- +0.1°
Correct Answer: +10°
Q14. Which sample preparation step is crucial for accurate polarimetric measurement?
- Using colored containers
- Filtering to remove particulates and degassing to remove bubbles
- Heating to boiling point
- Adding strong acids arbitrarily
Correct Answer: Filtering to remove particulates and degassing to remove bubbles
Q15. The sign of specific rotation indicates:
- Molecular weight
- Direction of rotation (positive for dextrorotatory, negative for levorotatory)
- Solubility in water
- Optical density
Correct Answer: Direction of rotation (positive for dextrorotatory, negative for levorotatory)
Q16. Optical rotatory dispersion (ORD) provides information about:
- Variation of optical rotation with wavelength
- Solubility across pH range
- Thermal stability
- Melting point
Correct Answer: Variation of optical rotation with wavelength
Q17. Which of the following is true for enantiomers?
- They have identical optical rotations
- They rotate plane-polarized light equally but in opposite directions
- They have different boiling points
- They have different chemical formulas
Correct Answer: They rotate plane-polarized light equally but in opposite directions
Q18. For pharmaceutical quality control, polarimetry is commonly used to:
- Determine the optical purity and concentration of chiral drugs
- Measure endotoxin levels
- Assess tablet hardness
- Determine particle size distribution
Correct Answer: Determine the optical purity and concentration of chiral drugs
Q19. If a measured rotation is −5° for a solution of known concentration and path length, the compound is:
- Dextrorotatory
- Levorotatory
- Achiral
- Racemic
Correct Answer: Levorotatory
Q20. Which factor does NOT affect the observed optical rotation?
- Concentration of the solution
- Path length of the sample cell
- Wavelength of light used
- Magnetic field of the laboratory
Correct Answer: Magnetic field of the laboratory
Q21. Enantiomeric excess (ee) is defined as:
- Difference in mass between enantiomers
- Percent difference between amounts of two enantiomers
- Sum of optical rotations of enantiomers
- Ratio of diastereomers
Correct Answer: Percent difference between amounts of two enantiomers
Q22. A polarimeter usually contains which of the following optical elements?
- Diffraction grating and monochromator
- Polarizer and analyzer
- Laser cavity and photomultiplier
- Interferometer and prism
Correct Answer: Polarizer and analyzer
Q23. In the presence of light-absorbing chiral compounds, which technique complements polarimetry for stereochemical analysis?
- Refractometry
- Circular dichroism (CD)
- Thermogravimetric analysis
- Atomic absorption
Correct Answer: Circular dichroism (CD)
Q24. If the specific rotation of a pure enantiomer is +50° and a sample shows +25°, the optical purity (%) is:
- 25%
- 50%
- 75%
- 100%
Correct Answer: 50%
Q25. Which pharmaceutical example often analyzed by polarimetry for purity is:
- Sucrose in syrups
- Sodium chloride in injections
- Paracetamol tablets
- Calcium carbonate suspensions
Correct Answer: Sucrose in syrups
Q26. A sample cell with length 2 dm is used. Observed rotation is +20° for 0.5 g/mL solution. Specific rotation is:
- +20°
- +10°
- +20°/(2×0.5) = +20°
- +20°/(2×0.5) = +20° — (check units)
Correct Answer: +20°/(2×0.5) = +20° — (check units)
Q27. Which is a common source of error in polarimetric measurements?
- Improper calibration with a standard
- Using distilled water as reference
- Measuring at standard temperature
- Using the correct path length
Correct Answer: Improper calibration with a standard
Q28. A racemic mixture shows what net optical rotation?
- Positive
- Negative
- Zero
- Undefined
Correct Answer: Zero
Q29. Optical rotation is an example of which macroscopic property related to molecular chirality?
- Scalar property
- Pseudo-scalar property
- Vector property
- Tensor property
Correct Answer: Pseudo-scalar property
Q30. For a dilute sugar solution, specific rotation remains constant if:
- Temperature, wavelength, and solvent are unchanged
- Concentration is doubled
- Path length is doubled
- Sample is heated to 100°C
Correct Answer: Temperature, wavelength, and solvent are unchanged
Q31. Which solvent effect can influence observed optical rotation?
- Hydrogen bonding and solvent polarity causing conformational changes
- Atmospheric pressure only
- Color of the solvent
- Solvent container shape
Correct Answer: Hydrogen bonding and solvent polarity causing conformational changes
Q32. Optical rotation is additive for:
- Non-interacting enantiomeric mixtures according to their concentrations
- Compounds that react chemically
- Only racemic mixtures
- Compounds with no chiral centers
Correct Answer: Non-interacting enantiomeric mixtures according to their concentrations
Q33. Which calibrant is commonly used to check a polarimeter?
- Glucose solution of known rotation
- 6000 ppm NaCl solution
- Pure ethanol
- Saturated sodium sulfate
Correct Answer: Glucose solution of known rotation
Q34. Optical rotation of a molecule arises due to:
- Presence of an achiral chromophore
- Asymmetry in spatial arrangement of atoms (chirality)
- High molecular weight only
- Presence of heavy metals
Correct Answer: Asymmetry in spatial arrangement of atoms (chirality)
Q35. Which is true for diastereomers regarding optical rotation?
- They have equal and opposite rotations
- They can have different magnitudes and signs of rotation
- They are always racemic
- They are optically inactive
Correct Answer: They can have different magnitudes and signs of rotation
Q36. If a pure enantiomer has [α] = +40° and a sample of unknown composition shows [α] = −10°, the sample composition indicates:
- Pure enantiomer of same sign
- Excess of opposite enantiomer
- Pure racemate
- No chiral compound present
Correct Answer: Excess of opposite enantiomer
Q37. Which advanced technique combines chromatography with polarimetric detection to separate and measure enantiomers?
- GC-MS
- Chiral HPLC with polarimetric or CD detector
- Flash chromatography
- Size exclusion chromatography
Correct Answer: Chiral HPLC with polarimetric or CD detector
Q38. In pharmaceutical formulations, optical rotation helps confirm:
- Presence and orientation of chiral active pharmaceutical ingredients (APIs)
- Tablet disintegration time
- Viscosity of suspensions
- pH of solution
Correct Answer: Presence and orientation of chiral active pharmaceutical ingredients (APIs)
Q39. Which is true about observed rotation α when concentration doubles and path length remains constant?
- α remains unchanged
- α doubles approximately for ideal solutions
- α halves
- α becomes zero
Correct Answer: α doubles approximately for ideal solutions
Q40. Optical rotation measurements can be used to monitor which type of chemical reaction in pharma?
- Racemization and resolution reactions
- Neutralization reactions only
- Crystallization without chirality change
- Combustion reactions
Correct Answer: Racemization and resolution reactions
Q41. What is the main limitation of polarimetry compared to chiral chromatography?
- Polarimetry provides structural elucidation
- Polarimetry cannot separate components in mixtures of chiral compounds
- Polarimetry is always more sensitive
- Polarimetry measures mass directly
Correct Answer: Polarimetry cannot separate components in mixtures of chiral compounds
Q42. For optical rotation measurements in aqueous solution, the solvent blank should be:
- Unrelated solvent
- Same solvent and treated the same way as sample
- Pure ethanol always
- Discarded and not used
Correct Answer: Same solvent and treated the same way as sample
Q43. Which property of light is essential for polarimetry?
- Polarization
- Coherence length only
- Speed in vacuum
- Temperature of source
Correct Answer: Polarization
Q44. A substance showing no optical rotation is necessarily:
- Achiral or racemic
- Always achiral only
- Always racemic only
- Optically active but undetectable
Correct Answer: Achiral or racemic
Q45. Which data from polarimetry can help determine stereochemical configuration when combined with other information?
- Specific rotation sign and magnitude compared with literature
- Only the color of solution
- Only the density of solution
- Boiling point data
Correct Answer: Specific rotation sign and magnitude compared with literature
Q46. How does path length influence observed rotation in Beer’s-like behavior for optical rotation?
- Observed rotation is inversely proportional to path length
- Observed rotation is directly proportional to path length
- Observed rotation is independent of path length
- Observed rotation is quadratically related to path length
Correct Answer: Observed rotation is directly proportional to path length
Q47. When reporting specific rotation in a lab report, which conditions must be specified?
- Temperature, wavelength, solvent, concentration, path length
- Only temperature
- Only instrument brand
- Only operator’s name
Correct Answer: Temperature, wavelength, solvent, concentration, path length
Q48. Optical rotation can be used in which pharmaceutical manufacturing control?
- Monitoring sugar content in oral syrups
- Measuring tablet porosity
- Determining capsule color
- Measuring dissolution pH
Correct Answer: Monitoring sugar content in oral syrups
Q49. Which statement about specific rotation of a compound recorded in literature is correct?
- It is absolute and never depends on solvent or temperature
- It is conditional and must include experimental conditions for comparison
- It is only valid for solids
- It is always positive
Correct Answer: It is conditional and must include experimental conditions for comparison
Q50. How can polarimetry contribute to ensuring drug safety and efficacy?
- By assessing stereochemical purity which can affect pharmacological activity and toxicity
- By measuring microbial contamination directly
- By replacing all chromatographic tests
- By determining tablet hardness only
Correct Answer: By assessing stereochemical purity which can affect pharmacological activity and toxicity
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