Raoult’s law MCQs With Answer

Raoult’s law MCQs With Answer

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Raoult’s law is a foundational concept in physical pharmacy describing how the vapor pressure of an ideal solution depends on component mole fractions. This concise MCQ set for B. Pharm students focuses on vapor pressure lowering, partial pressures, ideal versus non‑ideal solutions, activity and activity coefficients, colligative effects, and limitations of Raoult’s law in pharmaceutical formulations. Questions include derivations, numerical problems, and real‑world applications such as azeotrope behavior and solvent selection in drug processing. Clear explanations and calculations will build conceptual depth and exam readiness. Keywords: Raoult’s law, MCQs, vapor pressure, colligative properties, ideal solutions, activity coefficient, azeotrope, B. Pharm. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the basic mathematical statement of Raoult’s law for a component i in an ideal binary solution?

  • p_i = x_i + p_i°
  • p_i = x_i p_i°
  • p_i = p_i° / x_i
  • p_i = x_j p_j°

Correct Answer: p_i = x_i p_i°

Q2. For a solution of a non‑volatile solute in a volatile solvent, how does Raoult’s law predict the total vapor pressure?

  • Total pressure = p_solute + p_solvent where p_solute = x_solute p_solute°
  • Total pressure = x_solvent p_solvent°
  • Total pressure = p_solvent°
  • Total pressure = x_solute p_solvent°

Correct Answer: Total pressure = x_solvent p_solvent°

Q3. Relative lowering of vapor pressure (Δp/p°) for an ideal solution containing a non‑volatile solute equals which quantity?

  • x_solvent
  • x_solute
  • p_solution / p°
  • 1 / x_solute

Correct Answer: x_solute

Q4. Which of the following modifications accounts for non‑ideal behavior in Raoult’s law?

  • p_i = x_i p_i° / γ_i
  • p_i = γ_i x_i p_i°
  • p_i = x_i p_i° – γ_i
  • p_i = p_i° / (γ_i x_i)

Correct Answer: p_i = γ_i x_i p_i°

Q5. Pure solvent A has vapor pressure 24.0 torr at 25°C. If mole fraction of solute B is 0.02 in an ideal solution, what is the solution vapor pressure?

  • 24.48 torr
  • 23.52 torr
  • 24.00 torr
  • 0.48 torr

Correct Answer: 23.52 torr

Q6. In the expression a_i = γ_i x_i, a_i stands for:

  • Molar mass of component i
  • Activity of component i
  • Activity coefficient of component i
  • Mole fraction of component i in vapor

Correct Answer: Activity of component i

Q7. Which statement about an ideal solution is correct?

  • Intermolecular interactions A–A, B–B, and A–B are identical
  • A–B interactions are much stronger than A–A and B–B
  • A–B interactions are negligible
  • Only entropy drives mixing; enthalpy change is infinite

Correct Answer: Intermolecular interactions A–A, B–B, and A–B are identical

Q8. What is the vapor pressure of a binary ideal solution if p_A° = 100 torr, p_B° = 50 torr, x_A = 0.6 and x_B = 0.4?

  • 80 torr
  • 140 torr
  • 60 torr
  • 100 torr

Correct Answer: 80 torr

Q9. Which phenomenon indicates a positive deviation from Raoult’s law?

  • Observed vapor pressure lower than predicted
  • Observed vapor pressure higher than predicted
  • Solution freezes at higher temperature than pure solvent
  • No vapor forms above solution

Correct Answer: Observed vapor pressure higher than predicted

Q10. For dilute solutions of non‑volatile solute, which colligative property relation directly follows from Raoult’s law?

  • Osmotic pressure π = MRT
  • Freezing point depression ΔTf = Kf m
  • Surface tension increases with concentration
  • Viscosity decreases linearly with mole fraction

Correct Answer: Freezing point depression ΔTf = Kf m

Q11. A binary solution shows negative deviation from Raoult’s law. Which is most likely?

  • A–B interactions are stronger than A–A and B–B
  • A–B interactions are weaker than A–A and B–B
  • The solution forms a simple ideal mixture
  • The solution behaves as an ideal gas

Correct Answer: A–B interactions are stronger than A–A and B–B

Q12. Henry’s law is often used for which component in relation to Raoult’s law?

  • Solvent at high concentration
  • Solute at infinite dilution when not ideal
  • Both components equally in ideal solutions
  • Solid solute in crystalline phase

Correct Answer: Solute at infinite dilution when not ideal

Q13. Which unitless quantity measures deviation from ideality in vapor–liquid equilibrium?

  • Osmotic coefficient
  • Activity coefficient γ
  • Partial pressure p_i
  • Molar mass

Correct Answer: Activity coefficient γ

Q14. If x_solute = 0.05 in an ideal solution and p° = 200 mmHg, what is Δp (vapor pressure lowering)?

  • 10 mmHg
  • 190 mmHg
  • 5 mmHg
  • 0 mmHg

Correct Answer: 10 mmHg

Q15. Which of the following best defines an azeotrope relevant to Raoult’s law?

  • A mixture whose vapor composition equals its liquid composition at a given temperature
  • A mixture where Raoult’s law holds exactly for all compositions
  • A solid solution with constant melting point
  • A nonvolatile solute in an ideal solvent

Correct Answer: A mixture whose vapor composition equals its liquid composition at a given temperature

Q16. For a binary ideal solution, which relation holds between mole fraction in vapor (y_i) and in liquid (x_i)?

  • y_i = x_i p_i° / (x_A p_A° + x_B p_B°)
  • y_i = x_i / p_i°
  • y_i = p_i° / x_i
  • y_i = x_i + p_i°

Correct Answer: y_i = x_i p_i° / (x_A p_A° + x_B p_B°)

Q17. When is Raoult’s law strictly valid?

  • Only for ideal gases
  • For ideal liquid mixtures where intermolecular forces are equal
  • For ionic solutions with strong electrostatic interactions
  • Only at very high pressures

Correct Answer: For ideal liquid mixtures where intermolecular forces are equal

Q18. A solution contains 0.10 mol of non‑volatile solute in 0.90 mol solvent. If vapor pressure of pure solvent is 150 mmHg, what is vapor pressure of solution (ideal)?

  • 15 mmHg
  • 135 mmHg
  • 150 mmHg
  • 165 mmHg

Correct Answer: 135 mmHg

Q19. Which calculation uses Raoult’s law to determine boiling point elevation qualitatively?

  • ΔTb ∝ x_solute
  • ΔTb = pV work
  • ΔTb = gas constant R only
  • ΔTb ∝ surface tension

Correct Answer: ΔTb ∝ x_solute

Q20. In a system showing strong positive deviation, what happens to vapor composition compared to ideal case?

  • Vapor is richer in the component with stronger A–B interactions
  • Vapor pressure and vapor composition tend toward pure component values, often enriching more volatile component
  • Vapor composition equals liquid composition
  • No vapor forms

Correct Answer: Vapor pressure and vapor composition tend toward pure component values, often enriching more volatile component

Q21. For a solution where γ_solvent = 1.05 and x_solvent = 0.95, p° = 250 mmHg, what is p_solvent (approx)?

  • 250 mmHg
  • 249.4 mmHg
  • 248.1 mmHg
  • 252.5 mmHg

Correct Answer: 248.1 mmHg

Q22. How does addition of a non‑volatile solute affect the boiling point of a solvent?

  • Boiling point decreases
  • Boiling point increases
  • No change in boiling point
  • Boiling point becomes undefined

Correct Answer: Boiling point increases

Q23. Which experimental observation would directly indicate Raoult’s law violation?

  • Linear dependence of p on x
  • Measured partial pressure differs from x_i p_i° significantly
  • Monotonic vapor pressure lowering with solute concentration
  • Equal vapor and liquid compositions at azeotrope

Correct Answer: Measured partial pressure differs from x_i p_i° significantly

Q24. In pharmaceutical formulations, understanding Raoult’s law helps primarily with which process?

  • Tablet mechanical strength testing
  • Solvent selection for extraction and distillation
  • Color matching of coatings
  • Determining porosity of powders

Correct Answer: Solvent selection for extraction and distillation

Q25. Which expression relates osmotic pressure to colligative behavior (conceptually connected to Raoult’s law)?

  • πV = nRT
  • π = γ x p°
  • π = ΔT_f Kf
  • π = p_total – p_solvent

Correct Answer: πV = nRT

Q26. If a binary liquid shows ideal behavior at low concentration but deviates at high concentration, which approach is appropriate?

  • Apply Raoult’s law at all concentrations
  • Use Raoult’s law near infinite dilution and activity coefficients at higher concentration
  • Use only Henry’s law for both components
  • Assume vapor pressure independent of composition

Correct Answer: Use Raoult’s law near infinite dilution and activity coefficients at higher concentration

Q27. Which of the following is TRUE for an ideal dilute solution of a non‑volatile solute?

  • Δp/p° = molality of solute
  • Δp/p° ≈ mole fraction of solute
  • Activity coefficients are >>1
  • Vapor pressure increases with increasing solute

Correct Answer: Δp/p° ≈ mole fraction of solute

Q28. A pharmaceutical solvent mixture obeys Raoult’s law. If p_A° = 120 mmHg, p_B° = 60 mmHg and x_A = 0.25, what is partial pressure of A?

  • 30 mmHg
  • 45 mmHg
  • 75 mmHg
  • 120 mmHg

Correct Answer: 30 mmHg

Q29. Activity coefficients approaching unity indicate what behavior?

  • Strong non‑ideality
  • Near ideal solution behavior
  • Infinite dilution of solute
  • Solution is solid

Correct Answer: Near ideal solution behavior

Q30. Which is the correct expression for total pressure of a binary ideal solution?

  • P_total = x_A + x_B
  • P_total = x_A p_A° + x_B p_B°
  • P_total = p_A° p_B°
  • P_total = γ_A x_A p_A° + γ_B x_B p_B°

Correct Answer: P_total = x_A p_A° + x_B p_B°

Q31. Which parameter is necessary to use when Raoult’s law fails for the solute at low concentrations?

  • Henry’s law constant
  • Boiling point elevation constant
  • Molar refractivity
  • Partition coefficient

Correct Answer: Henry’s law constant

Q32. A mixture contains equal moles of A and B. If p_A° = 200 mmHg and p_B° = 50 mmHg, what is total pressure?

  • 125 mmHg
  • 250 mmHg
  • 150 mmHg
  • 100 mmHg

Correct Answer: 125 mmHg

Q33. In calculating vapor composition from Raoult’s law, which ratio gives mole fraction of A in vapor?

  • y_A = x_A p_A° / P_total
  • y_A = p_A° / x_A
  • y_A = x_A / p_A°
  • y_A = x_A + p_A°

Correct Answer: y_A = x_A p_A° / P_total

Q34. Which statement about colligative properties is correct in relation to Raoult’s law?

  • They depend on chemical identity of solute
  • They depend only on number of solute particles, linked to mole fraction
  • They are independent of concentration
  • They are directly given by p_i = γ_i x_i p_i°

Correct Answer: They depend only on number of solute particles, linked to mole fraction

Q35. For a dilute solution, Raoult’s law predicts Δp = x_solute p°. If p° = 500 mmHg and x_solute = 0.01, what is Δp?

  • 5 mmHg
  • 0.5 mmHg
  • 50 mmHg
  • 495 mmHg

Correct Answer: 5 mmHg

Q36. Which of the following is a limitation of using Raoult’s law directly in formulation design?

  • It cannot predict vapor pressure lowering
  • It ignores specific solute‑solvent interactions (non‑ideal behavior)
  • It is only valid for ionic solids
  • It requires knowledge of crystal structure

Correct Answer: It ignores specific solute‑solvent interactions (non‑ideal behavior)

Q37. If activity coefficient γ < 1 for a component, what does it imply about interactions?

  • Repulsive interactions dominate (positive deviation)
  • Attractive interactions stabilize the component (negative deviation)
  • No interactions present
  • Component is nonvolatile

Correct Answer: Attractive interactions stabilize the component (negative deviation)

Q38. A solution shows total vapor pressure lower than the ideal prediction. This is most consistent with:

  • Positive deviation
  • Negative deviation
  • Perfectly ideal behavior
  • Complete immiscibility

Correct Answer: Negative deviation

Q39. Which experimental method can be used to determine activity coefficients related to Raoult’s law?

  • Vapor pressure measurements vs composition
  • X‑ray diffraction of solids
  • pH titration only
  • Colorimetric assay

Correct Answer: Vapor pressure measurements vs composition

Q40. In an ideal binary mixture, if one component is much more volatile (p° much larger), what is expected in vapor above equimolar liquid?

  • Vapor enriched in less volatile component
  • Vapor enriched in more volatile component
  • Vapor composition equal to 50:50
  • No vapor forms

Correct Answer: Vapor enriched in more volatile component

Q41. When expressing Raoult’s law for solvent S and solute X where X is nonvolatile, which formula is correct for vapor pressure lowering Δp?

  • Δp = p°_S (1 – x_S)
  • Δp = p°_S x_S
  • Δp = x_S p°_X
  • Δp = p°_S / x_X

Correct Answer: Δp = p°_S (1 – x_S)

Q42. For a pharmaceutical distillation, why is knowledge of deviations from Raoult’s law important?

  • It affects design of separation steps and predicts azeotrope formation
  • It determines tablet disintegration time
  • It controls polymer molecular weight
  • It is only theoretical with no practical effect

Correct Answer: It affects design of separation steps and predicts azeotrope formation

Q43. If a volatile solute follows Henry’s law at low concentration, the relation between its partial pressure and mole fraction is:

  • p = k_H x
  • p = x p°
  • p = γ x p°
  • p = p° / x

Correct Answer: p = k_H x

Q44. Which condition ensures Raoult’s law for component A and Henry’s law for component B can both be applied in a mixture?

  • Both components are nonvolatile
  • One component behaves ideally as solvent (Raoult) while the other is at infinite dilution (Henry)
  • System temperature is absolute zero
  • Mixture is solid

Correct Answer: One component behaves ideally as solvent (Raoult) while the other is at infinite dilution (Henry)

Q45. Calculate mole fraction of solvent if vapor pressure of solution is 190 mmHg and p°_solvent = 200 mmHg (ideal).

  • 0.95
  • 0.90
  • 0.05
  • 1.05

Correct Answer: 0.95

Q46. In a pharmaceutical context, which real‑world factor can cause deviation from Raoult’s law?

  • Hydrogen bonding between solvent and solute
  • Identical molecular sizes and interactions
  • Absence of vapor phase
  • Extremely low temperatures only

Correct Answer: Hydrogen bonding between solvent and solute

Q47. For a binary mixture, which graph helps visualize Raoult’s law predictions and deviations?

  • P–x and y–x vapor–liquid equilibrium diagrams
  • pH titration curve
  • IR absorption spectrum
  • DSC thermal curve only

Correct Answer: P–x and y–x vapor–liquid equilibrium diagrams

Q48. If total pressure over ideal solution is given by P = x_A p_A° + x_B p_B°, what is dP/dx_A at constant temperature?

  • dP/dx_A = p_A° – p_B°
  • dP/dx_A = p_A° + p_B°
  • dP/dx_A = x_A p_A°
  • dP/dx_A = 0

Correct Answer: dP/dx_A = p_A° – p_B°

Q49. Which approach quantifies how far a real solution deviates from Raoult’s law for component i?

  • Measure the ratio p_i / (x_i p_i°) = γ_i
  • Use pH alone
  • Calculate molecular weight of solvent
  • Measure viscosity change only

Correct Answer: Measure the ratio p_i / (x_i p_i°) = γ_i

Q50. Why is the concept of activity important when applying Raoult’s law to pharmaceutical mixtures?

  • Activity replaces mole fraction to account for non‑ideal interactions, improving prediction of vapor pressures and colligative effects
  • Activity measures pH only
  • Activity is used to calculate drug potency exclusively
  • Activity is only relevant for solids

Correct Answer: Activity replaces mole fraction to account for non‑ideal interactions, improving prediction of vapor pressures and colligative effects

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