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Strong/weak electrolytes and acid-base concepts: NEET MCQs with answers

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

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easy

1. For the polyprotic acid phosphoric acid (H₃PO₄), the successive ionisation constants in aqueous solution are Kₐ₁, Kₐ₂, and Kₐ₃. Which of the following relationships correctly compares their magnitudes?

  1. Kₐ₁ < Kₐ₂ < Kₐ₃
  2. Kₐ₂ > Kₐ₁ > Kₐ₃
  3. Kₐ₁ = Kₐ₂ = Kₐ₃
  4. Kₐ₁ > Kₐ₂ > Kₐ₃
Show answer and explanation

Answer: (D) Kₐ₁ > Kₐ₂ > Kₐ₃

It is much more difficult to remove a positively charged proton (H⁺) from an increasingly negatively charged conjugate anion. Therefore, the successive dissociation constants of a polybasic acid decrease substantially in the order Kₐ₁ > Kₐ₂ > Kₐ₃.

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2. Consider the following reaction in liquid sulfur dioxide: SOCl₂ + Cs₂SO₃ ⇌ 2CsCl + 2SO₂. Under the solvent system concept for liquid SO₂, which species acts as the acid?

  1. CsCl
  2. Cs₂SO₃
  3. SOCl₂
  4. SO₂
Show answer and explanation

Answer: (C) SOCl₂

In liquid SO₂, autoionisation occurs as 2SO₂ ⇌ SO²⁺ + SO₃²⁻. Substances that increase the concentration of the characteristic cation (SO²⁺), such as SOCl₂ (giving SO²⁺ + 2Cl⁻), behave as acids, while sulfites that provide SO₃²⁻ act as bases.

hard

3. The ionisation constant of a weak base BOH is 1.8 × 10⁻⁵ at 298 K. What is the ratio of degree of ionisation (α₁ / α₂) when the solution is diluted from 0.18 M to 0.02 M?

  1. 1 : 3
  2. 3 : 1
  3. 1 : 9
  4. 9 : 1
Show answer and explanation

Answer: (A) 1 : 3

According to Ostwald's dilution law for a weak base where α ≪ 1, α = √(K_b / C). Therefore, α₁ / α₂ = √(C₂ / C₁) = √(0.02 / 0.18) = √(1 / 9) = 1 / 3.

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4. Consider the following reaction: B(OH)₃ + 2H₂O ⇌ [B(OH)₄]⁻ + H₃O⁺ Which of the following statements correctly accounts for the acidic behavior of boric acid B(OH)₃?

  1. It is a monobasic Lewis acid that accepts an OH⁻ ion from water, releasing H₃O⁺.
  2. It is a protic Brønsted acid that donates a proton to water.
  3. It is a tribasic Arrhenius acid that releases three H⁺ ions sequentially.
  4. It is an amphoteric oxide that reacts with both acids and bases by proton exchange.
Show answer and explanation

Answer: (A) It is a monobasic Lewis acid that accepts an OH⁻ ion from water, releasing H₃O⁺.

Boric acid is not a proton donor (protic acid). Instead, it acts as a Lewis acid by accepting a pair of electrons from the oxygen of an OH⁻ ion of water, thereby completing the octet of boron and releasing a hydronium ion into the solution.

easy

5. Given below are two statements: Statement I: A strong acid always has a weak conjugate base. Statement II: According to the Brønsted-Lowry concept, a base is a substance capable of donating an electron pair. In light of the above statements, choose the correct answer:

  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: (C) Statement I is true but Statement II is false.

Statement I is true because the stronger the acid, the weaker is its conjugate base. Statement II is false because the Brønsted-Lowry concept defines a base as a proton (H⁺) acceptor, while an electron-pair donor is a Lewis base.

hard

6. Given the following values of ionisation constants: K₁(H₂S) = 1.0 × 10⁻⁷ K₂(H₂S) = 1.2 × 10⁻¹³ Kₐ(HCN) = 4.0 × 10⁻¹⁰ Which of the following acid-base reactions has an equilibrium constant K > 1?

  1. HS⁻(aq) + HCN(aq) ⇌ H₂S(aq) + CN⁻(aq)
  2. CN⁻(aq) + H₂S(aq) ⇌ HCN(aq) + HS⁻(aq)
  3. S²⁻(aq) + HCN(aq) ⇌ HS⁻(aq) + CN⁻(aq)
  4. Both (b) and (c) are reactions where K > 1
Show answer and explanation

Answer: (D) Both (b) and (c) are reactions where K > 1

The equilibrium constant for an acid-base proton transfer is K = Kₐ(reacting acid) / Kₐ(product acid). For CN⁻ + H₂S ⇌ HCN + HS⁻: K = K₁(H₂S) / Kₐ(HCN) = (1.0 × 10⁻⁷) / (4.0 × 10⁻¹⁰) = 250 > 1. For S²⁻ + HCN ⇌ HS⁻ + CN⁻: K = Kₐ(HCN) / K₂(H₂S) = (4.0 × 10⁻¹⁰) / (1.2 × 10⁻¹³) = 3.33 × 10³ > 1. However, since single correct option format requires one best choice, let us evaluate the option: the option explicitly states 'Both (b) and (c) are reactions where K > 1', making it the uniquely correct answer.

hard

7. A monobasic weak acid HA has an ionisation constant Kₐ = 1.6 × 10⁻⁵ at 25 °C. At what concentration of the acid will its degree of ionisation be 4.0%?

  1. 0.040 M
  2. 0.010 M
  3. 0.025 M
  4. 0.001 M
Show answer and explanation

Answer: (B) 0.010 M

Degree of ionisation α = 4.0% = 0.04. From the relation Kₐ = Cα² / (1 - α), and since α is small, Kₐ ≈ Cα². Therefore, C = Kₐ / α² = (1.6 × 10⁻⁵) / (0.04)² = (1.6 × 10⁻⁵) / (1.6 × 10⁻³) = 1.0 × 10⁻² M = 0.010 M.

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8. Which of the following acts as a Lewis acid without possessing an incomplete octet in its central atom?

  1. SiF₄
  2. AlCl₃
  3. BF₃
  4. BeCl₂
Show answer and explanation

Answer: (A) SiF₄

SiF₄ has a complete octet around the central Si atom (8 valence electrons). However, it behaves as a Lewis acid by expanding its octet using vacant 3d-orbitals to accept electron pairs (e.g., forming [SiF₆]²⁻). In contrast, BF₃, AlCl₃, and BeCl₂ act as Lewis acids due to an incomplete octet.

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9. Which of the following orders represents the correct increasing order of acidic strength among the hydrides of Group 16 elements in aqueous solution?

  1. H₂Te < H₂Se < H₂S < H₂O
  2. H₂S < H₂O < H₂Se < H₂Te
  3. H₂O < H₂S < H₂Se < H₂Te
  4. H₂O < H₂Te < H₂Se < H₂S
Show answer and explanation

Answer: (C) H₂O < H₂S < H₂Se < H₂Te

Down group 16, as atomic size increases from O to Te, the H–E bond length increases and the bond dissociation enthalpy decreases significantly. Lower bond strength facilitates easier release of H⁺ in aqueous solution, making H₂Te the strongest acid and H₂O the weakest: H₂O < H₂S < H₂Se < H₂Te.

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10. A weak monobasic acid HX has Kₐ = 1.6 × 10⁻⁵. At what molar concentration of HX will its percentage ionisation be exactly 2%?

  1. 0.01 M
  2. 0.02 M
  3. 0.08 M
  4. 0.04 M
Show answer and explanation

Answer: (D) 0.04 M

Percentage ionisation = 2%, so α = 0.02. Using Ostwald's dilution law for weak electrolytes (α ≪ 1): Kₐ = Cα². Therefore, C = Kₐ / α² = (1.6 × 10⁻⁵) / (0.02)² = (1.6 × 10⁻⁵) / (4.0 × 10⁻⁴) = 0.04 M.

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11. Which of the following statements correctly explains why BF₃ behaves as a Lewis acid whereas NF₃ does not?

  1. Boron has a lone pair of electrons whereas nitrogen has an incomplete octet
  2. The B-F bond is non-polar whereas the N-F bond is highly ionic
  3. Boron has vacant 3d-orbitals available for bonding, whereas nitrogen lacks d-orbitals
  4. Boron has an incomplete octet with a vacant 2p-orbital, whereas nitrogen has a complete octet with a lone pair
Show answer and explanation

Answer: (D) Boron has an incomplete octet with a vacant 2p-orbital, whereas nitrogen has a complete octet with a lone pair

In BF₃, the central boron atom has only six valence electrons (an incomplete octet) and a vacant 2p-orbital, allowing it to accept an electron pair. In contrast, nitrogen in NF₃ has a completed octet and a lone pair, so it cannot act as a Lewis acid.

easy

12. What is the conjugate acid of bicarbonate ion (HCO₃⁻) and its conjugate base, respectively?

  1. CO₃²⁻ and H₂CO₃
  2. H₂CO₃ and OH⁻
  3. H⁺ and CO₃²⁻
  4. H₂CO₃ and CO₃²⁻
Show answer and explanation

Answer: (D) H₂CO₃ and CO₃²⁻

A conjugate acid is formed by adding a proton: HCO₃⁻ + H⁺ → H₂CO₃. A conjugate base is formed by removing a proton: HCO₃⁻ - H⁺ → CO₃²⁻. Thus, the conjugate acid and base are H₂CO₃ and CO₃²⁻, respectively.

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