NEETAceToday's NEET Daily

Physical equilibria and Henry’s law: NEET MCQs with answers

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

Practise Equilibrium freeAll Equilibrium questions

Free with a Google sign-in; NEET Daily needs no sign-in at all.

easy

1. A gas 'G' reacts chemically with water according to: G(g) + H₂O(l) ⇌ H⁺(aq) + G⁻(aq). Which of the following statements best explains why Henry's law in its simple form (p = K_H · x) fails for such a gas?

  1. The gas becomes completely insoluble in water
  2. The molecular state of the gas changes upon dissolution due to dissociation
  3. The kinetic energy of the gas molecules increases sharply
  4. Henry's law is only applicable to noble gases
Show answer and explanation

Answer: (B) The molecular state of the gas changes upon dissolution due to dissociation

One of the essential conditions for the validity of Henry's law is that the solute gas must not undergo any chemical reaction, association, or dissociation in the solvent. When a gas ionises or reacts with the solvent, the simple proportionality between partial pressure and total dissolved concentration fails.

medium

2. Given below are two statements: One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Henry's law fails to describe the solubility of hydrogen chloride gas in water accurately. Reason (R): Hydrogen chloride dissociates into H⁺(aq) and Cl⁻(aq) ions in aqueous solution. In light of the above statements, choose the correct answer:

  1. Both (A) and (R) are true and (R) is the correct explanation of (A)
  2. Both (A) and (R) are true but (R) is not the correct explanation of (A)
  3. (A) is true but (R) is false
  4. (A) is false but (R) is true
Show answer and explanation

Answer: (A) Both (A) and (R) are true and (R) is the correct explanation of (A)

Henry's law applies strictly to gases that do not undergo chemical reaction, association, or dissociation in the solvent. Since HCl completely ionizes in water according to HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq), its solubility significantly exceeds the prediction of Henry's law.

hard

3. Which of the following physical equilibrium systems exhibits an equilibrium state where the equilibrium constant expression in terms of concentration simplifies to a constant ratio equal to the distribution or partition coefficient?

  1. Ice ⇌ Liquid water
  2. Saturated NaCl(aq) ⇌ Solid NaCl
  3. Solid camphor ⇌ Camphor vapour
  4. I₂ (aqueous layer) ⇌ I₂ (carbon tetrachloride layer)
Show answer and explanation

Answer: (D) I₂ (aqueous layer) ⇌ I₂ (carbon tetrachloride layer)

When a solute distributes between two immiscible liquid solvents at a constant temperature, dynamic physical equilibrium is reached: Solute(solvent 1) ⇌ Solute(solvent 2). The equilibrium constant is expressed as K = [Solute in solvent 2] / [Solute in solvent 1], known as the Nernst distribution law or partition coefficient.

medium

4. Given below are two statements: Statement I: A closed vessel containing liquid water at 100 °C and 1.013 bar exhibits an equilibrium where the rate of evaporation equals the rate of condensation. Statement II: The equilibrium constant for a liquid-vapour physical equilibrium is equal to the equilibrium vapour pressure of the liquid at that temperature. In the light of the above statements, choose the most appropriate answer:

  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
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

At the normal boiling point (100 °C at 1.013 bar), dynamic equilibrium is established between water and its vapour, where rate of evaporation equals rate of condensation. For H₂O(l) ⇌ H₂O(g), the equilibrium constant in terms of partial pressure is K_p = p(H₂O, g), which is equal to the vapour pressure of the liquid.

easy

5. To avoid the painful and life-threatening condition known as 'bends', deep-sea divers use breathing gas tanks filled with air diluted with which of the following gases, and what is the underlying physical basis?

  1. Hydrogen, because it diffuses much faster through cellular membranes
  2. Helium, because it has very low solubility in blood due to a high Henry's law constant
  3. Argon, because it is chemically completely inert in the human body
  4. Carbon dioxide, because it maintains the pH balance of the blood during decompression
Show answer and explanation

Answer: (B) Helium, because it has very low solubility in blood due to a high Henry's law constant

According to NCERT, scuba divers use air diluted with helium (11.7% He, 56.2% N₂, 32.1% O₂) to avoid bends because helium has very low solubility in blood (high K_H) compared to nitrogen, minimizing bubble formation during decompression.

hard

6. For a liquid-gas system at constant temperature, if a gas follows Henry's law (m = k · p, where m is mass dissolved and p is pressure), how does the volume of dissolved gas at the pressure of measurement change with pressure?

  1. It increases linearly with pressure
  2. It decreases inversely with pressure
  3. It increases quadratically with pressure
  4. It remains independent of pressure
Show answer and explanation

Answer: (D) It remains independent of pressure

According to Boyle's law, V = nRT/p. Since n is proportional to mass m, which is directly proportional to pressure p according to Henry's law (n ∝ p), the ratio n/p remains constant at constant temperature. Hence, the volume of dissolved gas measured at the pressure of measurement is independent of pressure.

easy

7. Consider the dissolution of sugar in water at a constant temperature. When the solution reaches saturation, which of the following is true?

  1. Concentration of sugar in the solution keeps increasing
  2. Rate of dissolution > Rate of crystallisation
  3. Dissolution stops completely at the molecular level
  4. Rate of dissolution = Rate of crystallisation
Show answer and explanation

Answer: (D) Rate of dissolution = Rate of crystallisation

At dynamic equilibrium between solid solute and solution, the rate of dissolution of solute equals the rate of crystallisation from solution. The processes continue at equal rates, keeping concentration constant.

medium

8. The solubility of gases in liquids:

  1. First increases then decreases always
  2. Decreases with increase in temperature
  3. Increases with temperature
  4. Is independent of temperature
Show answer and explanation

Answer: (B) Decreases with increase in temperature

Dissolution of gases is exothermic; by Le Chatelier's principle solubility decreases as temperature rises. Aquatic life is more comfortable in cold water.

NCERT Class 11

medium

9. For the sublimation equilibrium of ammonium chloride in a closed vessel: NH₄Cl(s) ⇌ NH₃(g) + HCl(g), if the equilibrium pressure is P at a temperature T, what is the value of K_p?

  1. P
  2. P / 2
  3. P² / 4
  4. P²
Show answer and explanation

Answer: (C) P² / 4

Since 1 mole of NH₄Cl produces 1 mole of NH₃ and 1 mole of HCl, the partial pressures of the two gases at equilibrium are equal: p(NH₃) = p(HCl) = P / 2. Therefore, K_p = p(NH₃) · p(HCl) = (P / 2) · (P / 2) = P² / 4.

medium

10. A liquid boils when its vapour pressure becomes equal to:

  1. Zero
  2. Its critical pressure
  3. Half the atmospheric pressure
  4. The external (atmospheric) pressure
Show answer and explanation

Answer: (D) The external (atmospheric) pressure

At the boiling point the vapour pressure of the liquid equals the external pressure. At 1 atm this is the normal boiling point.

NCERT Class 11

medium

11. Given below are two statements: Statement I: The vapour pressure of a pure liquid in a closed vessel depends only on temperature and is independent of the amount of liquid present. Statement II: As temperature increases, the fraction of molecules with kinetic energy high enough to escape the liquid surface increases. In light of the above statements, choose the correct answer from the options given below:

  1. Both Statement I and Statement II are true
  2. Both Statement I and Statement II are false
  3. Statement I is true but Statement II is false
  4. Statement I is false but Statement II is true
Show answer and explanation

Answer: (A) Both Statement I and Statement II are true

The vapour pressure of a liquid is an intensive property that depends solely on the nature of the liquid and the temperature, not on the surface area or volume of liquid (provided both phases coexist). Higher temperature gives more molecules the threshold kinetic energy to escape into the vapour phase, increasing equilibrium vapour pressure.

medium

12. For the solid-vapour equilibrium of iodine in a closed vessel: I₂(s) ⇌ I₂(g), which of the following is an intensive property that remains constant at a given temperature once equilibrium is attained?

  1. Total mass of solid iodine
  2. Intensity of the violet colour of the vapour
  3. Volume of the container
  4. Total moles of iodine in the system
Show answer and explanation

Answer: (B) Intensity of the violet colour of the vapour

At equilibrium, the concentration of I₂(g) becomes constant, which renders the intensity of the purple/violet colour constant. The colour intensity reflects concentration (an intensive property), whereas total mass depends on initial amounts.

Keep going with Equilibrium. All 601 questions with instant answers, your mistakes saved for revision and your accuracy by topic.
Start practising

Other topics in Equilibrium