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Ideal/non-ideal solutions and vapour-pressure composition plots: NEET MCQs with answers

90 questions on this topic of Solutions (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Given below are two statements: Statement I: An ideal solution can be separated completely into its pure components by fractional distillation. Statement II: Azeotropic mixtures boil at a constant temperature and distill over without any change in composition.

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Ideal solutions do not form azeotropes and have different compositions in liquid and vapour phases over the entire range, allowing complete separation by fractional distillation. Azeotropes are constant boiling mixtures whose vapour has the same composition as the liquid.

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2. A mixture of phenol and aniline shows negative deviation from Raoult's law primarily because:

  1. Phenol molecules dissociate completely into phenoxide ions in aniline.
  2. The dipolar attraction between aniline molecules is completely destroyed by non-polar phenol.
  3. An intermolecular hydrogen bond is formed between the phenolic proton and nitrogen atom of aniline.
  4. The entropy of mixing becomes negative due to chemical complexation.
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Answer: (C) An intermolecular hydrogen bond is formed between the phenolic proton and nitrogen atom of aniline.

The intermolecular hydrogen bonding between the phenolic -OH group and the nitrogen of the -NH₂ group of aniline is stronger than the respective self-associated hydrogen bonding in pure phenol and pure aniline. This stronger A-B interaction decreases escaping tendency, causing negative deviation.

hard

3. According to Konovalov's rule for an ideal or non-azeotropic zeotropic solution of components A and B, if pure component A is more volatile than pure component B (p_A° > p_B°), then at any composition:

  1. The vapour phase is enriched with component B when x_A exceeds 0.5.
  2. The mole fraction of B in the vapour is always greater than its mole fraction in the liquid (y_B > x_B).
  3. The mole fractions in both phases are strictly identical (y_A = x_A).
  4. The mole fraction of A in the vapour is always greater than its mole fraction in the liquid (y_A > x_A).
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Answer: (D) The mole fraction of A in the vapour is always greater than its mole fraction in the liquid (y_A > x_A).

According to Konovalov's rule, the vapour phase is always richer in the more volatile component compared to the liquid phase with which it is in equilibrium. Since p_A° > p_B°, component A is more volatile, and therefore y_A > x_A.

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4. Which of the following pairs of liquids exhibits maximum boiling azeotropic behaviour?

  1. Water (boiling point 373 K) and Ethanol (boiling point 351.3 K)
  2. Water (boiling point 373 K) and Nitric acid (boiling point 359 K)
  3. Benzene (boiling point 353 K) and Toluene (boiling point 383.6 K)
  4. Carbon tetrachloride (boiling point 350 K) and Chloroform (boiling point 334 K)
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Answer: (B) Water (boiling point 373 K) and Nitric acid (boiling point 359 K)

A mixture of water and nitric acid (approximately 68% HNO₃ by mass) shows large negative deviation from Raoult's law and forms a maximum boiling azeotrope boiling at 393.5 K. Water + ethanol forms a minimum boiling azeotrope, while benzene + toluene forms an ideal solution.

easy

5. When 100 mL of pure liquid X is mixed with 100 mL of pure liquid Y at 25 °C, the total volume of the resulting mixture is found to be 203 mL. What conclusion can be drawn regarding the forces of interaction and the nature of the solution?

  1. X-Y interactions are stronger than X-X and Y-Y; the solution shows negative deviation.
  2. X-Y interactions are identical to X-X and Y-Y; the solution is ideal.
  3. X-Y interactions are weaker than X-X and Y-Y; the solution shows positive deviation.
  4. X-Y interactions are stronger than X-X and Y-Y; the solution shows positive deviation.
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Answer: (C) X-Y interactions are weaker than X-X and Y-Y; the solution shows positive deviation.

Since the final volume (203 mL) is greater than the sum of initial volumes (200 mL), Δ_mixV > 0. An increase in volume on mixing indicates that the attractive forces between unlike molecules (X-Y) are weaker than those between like molecules (X-X and Y-Y), characteristic of positive deviation from Raoult's law.

hard

6. For an ideal solution of two volatile components A and B, a plot of (1 / P_total) versus y_A (mole fraction of A in vapour phase) yields a straight line. What is the value of the slope of this line?

  1. (1 / P_A°) - (1 / P_B°)
  2. (1 / P_B°) - (1 / P_A°)
  3. P_A° - P_B°
  4. (P_B° - P_A°) / (P_A° · P_B°)
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Answer: (A) (1 / P_A°) - (1 / P_B°)

From Raoult's and Dalton's laws, 1 / P_total = y_A / P_A° + y_B / P_B° = y_A / P_A° + (1 - y_A) / P_B° = (1 / P_B°) + y_A · [(1 / P_A°) - (1 / P_B°)]. Plotting 1 / P_total vs y_A gives a straight line with slope equal to (1 / P_A°) - (1 / P_B°).

easy

7. Which of the following binary liquid mixtures shows positive deviation from Raoult's law?

  1. Carbon tetrachloride + Chloroform
  2. Benzene + Toluene
  3. Acetone + Aniline
  4. Carbon tetrachloride + Methanol
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Answer: (D) Carbon tetrachloride + Methanol

In pure methanol, extensive intermolecular hydrogen bonding exists. Addition of non-polar carbon tetrachloride molecules inserts them between methanol molecules, breaking hydrogen bonds and weakening attractive forces (A-B < A-A), which results in a positive deviation from Raoult's law.

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8. On a vapour pressure-composition diagram for an ideal binary solution at a constant temperature, what shape is observed for the plot of total vapour pressure (P_total) versus the mole fraction of component A in the vapour phase (y_A)?

  1. Linear with a positive slope
  2. Linear with a negative slope
  3. Curved (non-linear) concave downwards
  4. Curved (non-linear) concave upwards
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Answer: (D) Curved (non-linear) concave upwards

The total vapour pressure plotted against the liquid mole fraction (x_A) is a straight line: P = P_B° + (P_A° - P_B°)x_A. However, against the vapour mole fraction (y_A), the relation is 1/P = y_A/P_A° + (1 - y_A)/P_B°, which is non-linear and curves upwards.

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9. Two volatile liquids A and B form an ideal solution at 300 K. The vapour pressures of pure A and pure B are 120 mm Hg and 180 mm Hg, respectively. If an equimolar liquid mixture of A and B is taken, what is the mole fraction of component B in the vapour phase at equilibrium?

  1. 0.60
  2. 0.50
  3. 0.40
  4. 0.75
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Answer: (A) 0.60

For an equimolar mixture, x_A = 0.5 and x_B = 0.5. The partial vapour pressures are p_A = 120 × 0.5 = 60 mm Hg and p_B = 180 × 0.5 = 90 mm Hg. The total pressure P_total = 60 + 90 = 150 mm Hg. Therefore, the mole fraction of B in vapour phase is y_B = p_B / P_total = 90 / 150 = 0.60.

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10. Given below are two statements: Statement I: Fractional distillation cannot separate the components of an azeotropic mixture. Statement II: In an azeotropic mixture, the composition of the liquid phase and vapour phase at equilibrium are identical. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct, and Statement II is the correct explanation of Statement I.
  2. Both Statement I and Statement II are correct, but Statement II is NOT the correct explanation of Statement I.
  3. Statement I is correct but Statement II is incorrect.
  4. Statement I is incorrect but Statement II is correct.
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Answer: (A) Both Statement I and Statement II are correct, and Statement II is the correct explanation of Statement I.

Fractional distillation relies on the difference in composition between the liquid and vapour phases. Because an azeotrope boils with identical compositions in both phases (y_i = x_i), further separation by fractional distillation is impossible.

easy

11. Which of the following pairs of solutions will show behaviour closest to Raoult's law over the entire range of concentration?

  1. Chlorobenzene + Bromobenzene
  2. Ethanol + Water
  3. Chloroform + Acetone
  4. Nitric acid + Water
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Answer: (A) Chlorobenzene + Bromobenzene

Chlorobenzene and bromobenzene are nearly identical in molecular size, structure, and polarity. The intermolecular attractive forces between chlorobenzene-chlorobenzene, bromobenzene-bromobenzene, and chlorobenzene-bromobenzene are almost identical, making it an ideal solution obeying Raoult's law.

hard

12. At a certain temperature, liquids A and B form an ideal solution. The vapour pressure of pure A is 200 Torr and that of pure B is 100 Torr. The vapour in equilibrium with a liquid mixture contains 80 mol% of A. What is the total vapour pressure of the system?

  1. 150 Torr
  2. 160 Torr
  3. 166.7 Torr
  4. 180 Torr
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Answer: (C) 166.7 Torr

We are given y_A = 0.8 and y_B = 0.2. Using the relationship 1 / P_total = y_A / P_A° + y_B / P_B°: 1 / P_total = 0.8 / 200 + 0.2 / 100 = 0.004 + 0.002 = 0.006 Torr⁻¹. Therefore, P_total = 1 / 0.006 = 1000 / 6 = 166.7 Torr.

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