Vapour pressure MCQs With Answer

Vapour pressure MCQs With Answer are essential study aids for B. Pharm students preparing for physical pharmacy and pharmaceutical analysis. This concise, keyword-rich introduction covers core concepts like vapour pressure, Raoult’s law, Clausius–Clapeyron, activity coefficient, and vapour–liquid equilibrium with pharmaceutical relevance. Focused on understanding vapour pressure lowering, azeotropes, measurement techniques (isoteniscope, manometer), and effects on drug stability and formulation, these MCQs reinforce theory and problem-solving. Each question links thermodynamic principles to practical pharmaceutical scenarios to build exam confidence. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is vapour pressure of a pure liquid at equilibrium?

• The pressure exerted by the liquid on the container walls
• The partial pressure of vapor in equilibrium with its liquid at a given temperature
• The atmospheric pressure required to boil the liquid
• The pressure at which the liquid solidifies

Correct Answer: The partial pressure of vapor in equilibrium with its liquid at a given temperature

Q2. How does vapour pressure of a liquid change with temperature?

• It decreases with increasing temperature
• It remains constant regardless of temperature
• It increases with increasing temperature
• It first decreases then increases

Correct Answer: It increases with increasing temperature

Q3. Which equation relates the temperature dependence of vapour pressure to enthalpy of vaporization?

• Raoult’s law
• Antoine equation
• Clausius–Clapeyron equation
• Henry’s law

Correct Answer: Clausius–Clapeyron equation

Q4. Raoult’s law for an ideal binary solution states:

• P_total = P_A^0 + P_B^0
• P_A = x_A × P_A^0 and P_B = x_B × P_B^0
• P_total = x_A + x_B
• P_A = P_B for ideal solutions

Correct Answer: P_A = x_A × P_A^0 and P_B = x_B × P_B^0

Q5. Vapour pressure lowering on adding a non-volatile solute is described by:

• ΔP = x_solvent × P°
• ΔP = x_solute × P° (approx. for dilute solutions)
• ΔP = P_total − P°
• ΔP = Kb × m

Correct Answer: ΔP = x_solute × P° (approx. for dilute solutions)

Q6. Which is a colligative property related directly to vapour pressure?

• Surface tension
• Vapour pressure lowering
• Viscosity
• Refractive index

Correct Answer: Vapour pressure lowering

Q7. A positive deviation from Raoult’s law indicates:

• Stronger A–B interactions than A–A or B–B
• Weaker A–B interactions leading to higher vapour pressure
• Ideal behaviour
• Formation of an azeotrope with lower boiling point

Correct Answer: Weaker A–B interactions leading to higher vapour pressure

Q8. A negative deviation from Raoult’s law is caused by:

• Weaker A–B interactions and increased vaporisation
• Stronger A–B interactions that reduce vapour pressure
• Presence of a non-volatile solute only
• Temperature decrease only

Correct Answer: Stronger A–B interactions that reduce vapour pressure

Q9. An azeotrope is:

• A solution that obeys Raoult’s law perfectly
• A mixture whose liquid and vapour phases have the same composition at boiling
• A non-volatile solute dissolved in a solvent
• A solution with constant activity coefficients

Correct Answer: A mixture whose liquid and vapour phases have the same composition at boiling

Q10. Which instrument is commonly used to measure equilibrium vapour pressure of liquids in the lab?

• Spectrophotometer
• Isoteniscope
• Chromatograph
• Polarimeter

Correct Answer: Isoteniscope

Q11. The Antoine equation is used to:

• Calculate activity coefficients
• Estimate vapour pressure as a function of temperature
• Predict freezing point depression
• Measure osmotic pressure

Correct Answer: Estimate vapour pressure as a function of temperature

Q12. For a pure liquid in a closed container at equilibrium, vapour pressure depends primarily on:

• The amount of liquid present (mass)
• The container volume
• The temperature
• The shape of the container

Correct Answer: The temperature

Q13. In vapour–liquid equilibrium, Dalton’s law implies:

• Total pressure equals the highest partial pressure only
• Total pressure equals the sum of partial pressures of all vapours
• Partial pressures are independent of composition
• Vapour behaves non-ideally always

Correct Answer: Total pressure equals the sum of partial pressures of all vapours

Q14. Vapor pressure is a key factor in pharmaceutical formulation because it influences:

• Electrochemical potential only
• Evaporation, stability of volatile drugs, and packaging requirements
• Only the color of the formulation
• Melting point of excipients only

Correct Answer: Evaporation, stability of volatile drugs, and packaging requirements

Q15. The integrated Clausius–Clapeyron equation is often written as:

• ln P = −ΔHvap / (R T) + C
• P = x × P°
• ΔTb = Kb × m
• P = A × e^(−B/T)

Correct Answer: ln P = −ΔHvap / (R T) + C

Q16. Fugacity is best described as:

• A measure of the mechanical strength of a liquid
• The effective pressure that replaces real pressure in non-ideal systems
• The true vapour pressure of an ideal gas only
• Always equal to atmospheric pressure

Correct Answer: The effective pressure that replaces real pressure in non-ideal systems

Q17. Which law governs the solubility of a gas in a liquid at low concentrations relative to vapour pressure concepts?

• Raoult’s law
• Henry’s law
• Clausius–Clapeyron law
• Van ‘t Hoff equation

Correct Answer: Henry’s law

Q18. Which statement about volatility is correct?

• Lower vapour pressure indicates higher volatility
• Higher vapour pressure indicates higher volatility
• Volatility is unrelated to vapour pressure
• Volatility depends only on molecular weight

Correct Answer: Higher vapour pressure indicates higher volatility

Q19. For an ideal solution, the activity coefficient (γ) equals:

• Zero
• One
• Infinite
• Equal to the mole fraction

Correct Answer: One

Q20. In a binary solution where B is non-volatile, the vapour pressure above the solution equals:

• x_B × P_B^0
• x_A × P_A^0 where A is the volatile solvent
• P_A^0 + P_B^0
• P_total independent of composition

Correct Answer: x_A × P_A^0 where A is the volatile solvent

Q21. Which of the following best describes the effect of a volatile solute on total vapour pressure of a solution?

• Total vapour pressure decreases irrespective of solute volatility
• Total vapour pressure equals sum of partial pressures of both components
• Total vapour pressure is only the vapour pressure of the solvent
• Total vapour pressure becomes zero

Correct Answer: Total vapour pressure equals sum of partial pressures of both components

Q22. The Clapeyron equation connects dP/dT to which thermodynamic quantities?

• Surface tension and viscosity
• Enthalpy change and change in molar volume between phases
• Molar mass and density only
• Concentration and dielectric constant

Correct Answer: Enthalpy change and change in molar volume between phases

Q23. Which technique is useful to measure sublimation vapour pressure of a solid drug?

• Isoteniscope only for liquids
• Knudsen effusion method
• Polarimetry
• UV-Vis spectroscopy

Correct Answer: Knudsen effusion method

Q24. Relative humidity is defined as:

• The ratio of actual vapour pressure to saturation vapour pressure at that temperature
• Total pressure divided by vapour pressure
• The incremental rise in vapour pressure with temperature
• Partial pressure of dry air

Correct Answer: The ratio of actual vapour pressure to saturation vapour pressure at that temperature

Q25. In a temperature–composition vapour–liquid diagram for a binary ideal mixture, the bubble point and dew point curves:

• Intersect at a single point only for azeotropic mixtures
• Are identical for all mixtures
• Never intersect
• Are irrelevant to Raoult’s law

Correct Answer: Intersect at a single point only for azeotropic mixtures

Q26. Which statement about boiling point and vapour pressure is correct?

• Boiling occurs when vapour pressure equals external pressure
• Boiling point decreases as vapour pressure increases at constant external pressure
• Boiling point is independent of vapour pressure
• Boiling occurs when vapour pressure is zero

Correct Answer: Boiling occurs when vapour pressure equals external pressure

Q27. Azeotropes that boil at a lower temperature than either pure component are called:

• Positive azeotropes (minimum-boiling)
• Negative azeotropes (maximum-boiling)
• Ideal azeotropes
• Non-azeotropic mixtures

Correct Answer: Positive azeotropes (minimum-boiling)

Q28. The activity (a_i) of a component in solution is defined as:

• a_i = γ_i × x_i where γ_i is activity coefficient
• a_i = x_i / γ_i
• a_i = P_i / P_total
• a_i = molecular weight × concentration

Correct Answer: a_i = γ_i × x_i where γ_i is activity coefficient

Q29. Which of the following causes a significant deviation from ideal vapour–liquid behaviour?

• Similar intermolecular forces between components
• Strong hydrogen bonding between unlike molecules
• Identical molecular sizes only
• Low temperatures only

Correct Answer: Strong hydrogen bonding between unlike molecules

Q30. The standard vapour pressure of a pure substance refers to:

• Its vapour pressure at standard temperature and pressure (0°C, 1 atm)
• Its vapour pressure at a specified temperature under equilibrium with the pure liquid
• Vapour pressure when mixed with any solute
• Vapour pressure at infinite dilution only

Correct Answer: Its vapour pressure at a specified temperature under equilibrium with the pure liquid

Q31. Which mathematical form approximates the relationship between vapour pressure and temperature for many liquids?

• Linear P vs T
• ln P vs 1/T (Clausius–Clapeyron linearity)
• P × T = constant
• P = constant independent of T

Correct Answer: ln P vs 1/T (Clausius–Clapeyron linearity)

Q32. For dilute ideal solutions with non-volatile solute, which statement is true for solvent vapour pressure?

• It increases proportionally with solute mole fraction
• It is equal to the vapour pressure of the pure solvent
• It decreases, approximately by ΔP = x_solute × P°
• It becomes independent of temperature

Correct Answer: It decreases, approximately by ΔP = x_solute × P°

Q33. Which parameter can be derived from a plot of ln P vs 1/T for a pure liquid?

• Activity coefficient
• Molar enthalpy (ΔHvap) of vaporization
• Boiling point at zero pressure
• Solubility product constant

Correct Answer: Molar enthalpy (ΔHvap) of vaporization

Q34. In pharmaceutical drying, vapour pressure difference drives:

• Osmosis only
• Evaporation and moisture removal from solids
• Chemical degradation exclusively
• Drug–excipient interactions only

Correct Answer: Evaporation and moisture removal from solids

Q35. A solution exhibits ideal behaviour when:

• Intermolecular interactions between unlike molecules equal those between like molecules
• All molecules are ionic
• Temperature is absolute zero
• One component is non-volatile always

Correct Answer: Intermolecular interactions between unlike molecules equal those between like molecules

Q36. Which of the following best describes the use of vapour pressure data in distillation design?

• It is unnecessary; viscosity data suffice
• Vapour pressures determine composition of vapor and number of stages required
• Vapour pressure only matters for solids
• It only affects corrosion rates

Correct Answer: Vapour pressures determine composition of vapor and number of stages required

Q37. The presence of a non-volatile solute in a solvent will cause which of the following changes?

• Increase in vapour pressure and decrease in boiling point
• Decrease in vapour pressure and elevation of boiling point
• No change in vapour pressure but increase in freezing point
• Complete evaporation of solvent

Correct Answer: Decrease in vapour pressure and elevation of boiling point

Q38. Which variable is negligible when integrating Clausius–Clapeyron for liquids to vapour transition?

• Molar volume of vapour
• Change in entropy
• Molar volume of liquid compared to vapour
• Enthalpy of vaporization

Correct Answer: Molar volume of liquid compared to vapour

Q39. Vapor pressure data are important for which of the following pharmaceutical processes?

• Sterility testing only
• Formulation stability, drying, lyophilization and packaging design
• Only for parenteral compatibility
• Only for tablet hardness testing

Correct Answer: Formulation stability, drying, lyophilization and packaging design

Q40. Which expression correctly gives total pressure of an ideal two-component vapour above a solution?

• P_total = x_A × P_A^0 − x_B × P_B^0
• P_total = x_A × P_A^0 + x_B × P_B^0
• P_total = P_A^0 × P_B^0
• P_total = P_A^0 / x_A

Correct Answer: P_total = x_A × P_A^0 + x_B × P_B^0

Q41. What is the expected behaviour of vapour pressure for an ideal solution at infinite dilution of component B?

• Partial pressure of B equals activity coefficient times mole fraction
• Partial pressure of B approaches its pure-component vapour pressure
• Partial pressure of B approaches x_B × P_B^0 and obeys Henry’s law or Raoult’s law depending on solute nature
• Partial pressure of B becomes zero

Correct Answer: Partial pressure of B approaches x_B × P_B^0 and obeys Henry’s law or Raoult’s law depending on solute nature

Q42. Which of the following statements about isoteniscope measurement is true?

• It measures freezing point depression only
• It determines vapour pressure over a liquid by balancing pressures in a capillary system
• It is used for spectroscopy
• It directly measures activity coefficients

Correct Answer: It determines vapour pressure over a liquid by balancing pressures in a capillary system

Q43. A drug with high vapour pressure at room temperature is likely to:

• Be highly stable to evaporation
• Sublime or evaporate and may require special packaging
• Have low volatility and remain solid indefinitely
• Be insoluble in all solvents

Correct Answer: Sublime or evaporate and may require special packaging

Q44. Which quantity is directly obtained from vapour–liquid equilibrium experimental data?

• Activity coefficients and relative volatilities
• Melting enthalpy only
• Optical rotation values
• Solid-state polymorph identity

Correct Answer: Activity coefficients and relative volatilities

Q45. In the context of vapour pressure, a liquid with a small enthalpy of vaporization will generally:

• Have a low vapour pressure at a given temperature
• Have a high vapour pressure at a given temperature
• Not evaporate at all
• Always form a negative azeotrope

Correct Answer: Have a high vapour pressure at a given temperature

Q46. Which relationship expresses Raoult’s law in terms of chemical potential and activity?

• μ_i = μ_i° + RT ln(x_i)
• a_i = γ_i × x_i and μ_i = μ_i° + RT ln(a_i)
• ΔG = −RT ln K only
• P_i = K × T

Correct Answer: a_i = γ_i × x_i and μ_i = μ_i° + RT ln(a_i)

Q47. Vapor pressure osmometry is a technique to:

• Measure vapour pressure lowering to determine molecular weight of solute
• Measure boiling point elevation directly
• Measure refractive index of solutions
• Evaluate solid-state stability only

Correct Answer: Measure vapour pressure lowering to determine molecular weight of solute

Q48. Which of the following increases when a solution shows positive deviation from Raoult’s law?

• Activity coefficients drop below unity
• Partial vapour pressures of components exceed ideal values
• Solution becomes non-volatile
• Boiling point increases relative to pure components

Correct Answer: Partial vapour pressures of components exceed ideal values

Q49. In pharmaceutical lyophilization, understanding vapour pressure of ice is crucial because:

• It determines the required shelf temperature and chamber pressure for sublimation
• It only affects color changes
• It is irrelevant to the process
• It controls the drug’s pH

Correct Answer: It determines the required shelf temperature and chamber pressure for sublimation

Q50. Which statement about vapour pressure and container headspace is correct?

• Vapour pressure cannot establish equilibrium with headspace
• At equilibrium, vapour partial pressure in headspace equals the substance’s equilibrium vapour pressure at that temperature
• Headspace pressure is always equal to atmospheric pressure regardless of vapour
• Headspace composition never reflects liquid composition

Correct Answer: At equilibrium, vapour partial pressure in headspace equals the substance’s equilibrium vapour pressure at that temperature

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