Ionic Equilibrium – Ostwald's Dilution Law and Concept of Acids and Bases: NEET MCQs with Answers

15 free questions on Ionic Equilibrium – Ostwald's Dilution Law and Concept of Acids and Bases for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Ostwald's Dilution Law and Concept of Acids and Bases: Weak and strong electrolytes, ionization of electrolytes, various concepts of acids and bases (Arrhenius. Bronsted - Lowry and Lewis) and their ionization, acid-base equilibria (including multistage ionization) and ionization constants

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Question 1

Previous-year question
Phosphoric acid ionises in three steps, with ionisation constants K_a1, K_a2 and K_a3 respectively, while K is its overall ionisation constant. Which of the following statements are true? [NEET 2025]
A. log K = log K_a1 + log K_a2 + log K_a3
B. H₃PO₄ is a stronger acid than H₂PO₄⁻ and HPO₄²⁻
C. K_a1 > K_a2 > K_a3
D. K_a1 = (K_a3 + K_a2)/2
  1. A and B only
  2. A and C only
  3. B, C and D only
  4. A, B and C only
Show answer
Answer: (D) A, B and C only
Explanation: Overall K = K_a1 × K_a2 × K_a3, so log K = log K_a1 + log K_a2 + log K_a3 (A true). Successive ionisation constants decrease (K_a1 > K_a2 > K_a3) because removing H⁺ from an increasingly negative ion is harder, so H₃PO₄ is a stronger acid than H₂PO₄⁻ and HPO₄²⁻ (B and C true). K_a1 is not the average of K_a2 and K_a3 (D false).

Question 2

Previous-year question
A weak acid HA is about 1% dissociated at equilibrium when its concentration is 0.1 mol L⁻¹. What is the correct value of its K_a at the same temperature? [NEET 2023 (Manipur)]
  1. 1 × 10⁻⁴
  2. 1 × 10⁻⁶
  3. 1 × 10⁻⁵
  4. 1 × 10⁻³
Show answer
Answer: (C) 1 × 10⁻⁵
Explanation: By Ostwald's dilution law, for small α, K_a = Cα². With C = 0.1 mol L⁻¹ and α = 0.01, K_a = 0.1 × (0.01)² = 0.1 × 10⁻⁴ = 1 × 10⁻⁵.

Question 3

Previous-year question
Which of the following species is NOT able to act both as a Brønsted acid and as a Brønsted base? [NEET (Odisha) 2019]
  1. HCO₃⁻
  2. NH₃
  3. HCl
  4. HSO₄⁻
Show answer
Answer: (C) HCl
Explanation: HCl can only donate a proton (forming Cl⁻); it does not accept a proton in aqueous chemistry, so it acts only as a Brønsted acid. HCO₃⁻ and HSO₄⁻ can lose a proton or gain one (forming H₂CO₃/H₂SO₄), and NH₃ can accept a proton (NH₄⁺) or lose one (NH₂⁻), so these are amphiprotic.

Question 4

Previous-year question
The conjugate bases of the Brønsted acids H₂O and HF are respectively [NEET 2019]
  1. OH⁻ and H₂F⁺
  2. H₃O⁺ and F⁻
  3. OH⁻ and F⁻
  4. H₃O⁺ and H₂F⁺
Show answer
Answer: (C) OH⁻ and F⁻
Explanation: A conjugate base is what remains after the acid loses one proton. H₂O − H⁺ = OH⁻ and HF − H⁺ = F⁻. The species H₃O⁺ and H₂F⁺ would be the conjugate acids, formed by gaining a proton.

Question 5

Previous-year question
Benzene is nitrated using a mixture of concentrated H₂SO₄ and HNO₃. If a large amount of KHSO₄ is added to the mixture, the rate of nitration will be [NEET 2016]
  1. faster
  2. slower
  3. unchanged
  4. doubled
Show answer
Answer: (B) slower
Explanation: KHSO₄ supplies HSO₄⁻, which by the common-ion effect suppresses the ionisation of H₂SO₄ and lowers the H⁺ concentration. Less H⁺ means less of the nitronium ion NO₂⁺ (formed from HNO₃ + 2H₂SO₄), which is the actual electrophile, so nitration slows down.

Question 6

Previous-year question
HgCl₂ and I₂, both dissolved in water containing I⁻ ions, form the pair of species: [2017]
  1. HgI₂, I₃⁻
  2. HgI₄²⁻, I₃⁻
  3. Hg₂I₂, I⁻
  4. HgI₂, I₃⁻
Show answer
Answer: (B) HgI₄²⁻, I₃⁻
Explanation: HgCl₂ + 4I⁻ → HgI₄²⁻ and I₂ + I⁻ → I₃⁻; so the species are HgI₄²⁻ and I₃⁻.

Question 7

Which of the following statements about the ions formed from an acid or a base in water is incorrect?
  1. A bare proton has a very small size and an intense electric field, so it cannot exist freely in water
  2. Unlike the proton, the hydroxyl ion stays unhydrated in water, so OH⁻(aq) denotes a free OH⁻ ion
  3. In aqueous solution H₃O⁺ is further hydrated to species such as H₅O₂⁺, H₇O₃⁺ and H₉O₄⁺
  4. The proton attaches to the oxygen of a water molecule through a lone pair, giving a trigonal pyramidal H₃O⁺ ion
Show answer
Answer: (B) Unlike the proton, the hydroxyl ion stays unhydrated in water, so OH⁻(aq) denotes a free OH⁻ ion
Explanation: The hydroxyl ion is also hydrated in water, giving species such as H₃O₂⁻, H₅O₃⁻ and H₇O₄⁻. The other statements are correct descriptions of the proton in water. The symbols H⁺(aq) and H₃O⁺(aq) are used interchangeably to mean a hydrated proton.
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Question 8

Which option correctly compares acid strength in the two sets and names the deciding factor for each? Set P: CH₄, NH₃, H₂O and HF. Set Q: H₂O and H₂S.
  1. P: HF < H₂O < NH₃ < CH₄, decided by size of A; Q: H₂O < H₂S, decided by H–A bond polarity
  2. P: CH₄ < NH₃ < H₂O < HF, decided by H–A bond polarity; Q: H₂O < H₂S, decided by H–A bond strength
  3. P: CH₄ < NH₃ < H₂O < HF, decided by size of A; Q: H₂S < H₂O, decided by electronegativity of A
  4. P: CH₄ < NH₃ < H₂O < HF, decided by bond strength; Q: H₂S < H₂O, decided by H–A bond polarity
Show answer
Answer: (B) P: CH₄ < NH₃ < H₂O < HF, decided by H–A bond polarity; Q: H₂O < H₂S, decided by H–A bond strength
Explanation: Within the same period, the polarity of the H–A bond decides acid strength, so acid strength rises as the electronegativity of A increases from C to F. On going down a group, H–A bond strength is the more important factor. The larger atom forms a weaker bond with hydrogen, so H₂S is a stronger acid than H₂O even though oxygen is the more electronegative element.

Question 9

Acid strength rises along the series NH₃ < H₂O < HF in a period, and also along HF < HCl < HBr < HI down a group. Which option correctly names the main factor in each case?
  1. In both series: rising molar mass of the acid, which makes loss of H⁺ easier
  2. Along the period: falling H–A bond strength as A becomes larger; down the group: rising polarity of the H–A bond
  3. Along the period: rising polarity of the H–A bond as A becomes more electronegative; down the group: falling H–A bond strength as A becomes larger
  4. In both series: rising polarity of the H–A bond, since electronegativity of A rises in both
Show answer
Answer: (C) Along the period: rising polarity of the H–A bond as A becomes more electronegative; down the group: falling H–A bond strength as A becomes larger
Explanation: Across a period the atom A stays of similar size while its electronegativity rises, so the H–A bond becomes more polar and H⁺ is released more easily. Down a group the size of A increases strongly, so the overlap with hydrogen weakens and the H–A bond becomes weaker. This effect outweighs the fall in bond polarity, which is why HF is the weakest acid of the halogen family even though its bond is the most polar.

Question 10

By comparing the strengths of their conjugate acids, arrange the anions C₂H₅O⁻, C₆H₅O⁻, CH₃COO⁻ and Cl⁻ in decreasing order of base strength.
  1. C₂H₅O⁻ > CH₃COO⁻ > C₆H₅O⁻ > Cl⁻
  2. Cl⁻ > CH₃COO⁻ > C₆H₅O⁻ > C₂H₅O⁻
  3. C₆H₅O⁻ > C₂H₅O⁻ > CH₃COO⁻ > Cl⁻
  4. C₂H₅O⁻ > C₆H₅O⁻ > CH₃COO⁻ > Cl⁻
Show answer
Answer: (D) C₂H₅O⁻ > C₆H₅O⁻ > CH₃COO⁻ > Cl⁻
Explanation: The stronger an acid, the weaker its conjugate base. Acid strength falls in the order HCl > CH₃COOH > C₆H₅OH > C₂H₅OH. Reversing this gives the base strength order, with ethoxide the strongest base and chloride almost without basic character.

Question 11

For phosphoric acid, Ka₁ > Ka₂ > Ka₃ by a large factor at each step. What is the main reason?
  1. Water is a poorer base towards H₃PO₄ than towards H₂PO₄⁻ and HPO₄²⁻
  2. Each later step removes a proton from a less stable anion, so that proton leaves more readily
  3. Each later step removes a proton from a more negatively charged species, which holds it more strongly
  4. The O–H bonds become stronger because phosphorus changes its oxidation state at each step
Show answer
Answer: (C) Each later step removes a proton from a more negatively charged species, which holds it more strongly
Explanation: The first proton leaves a neutral molecule, the second leaves a singly charged anion, and the third leaves a doubly charged anion. Greater negative charge attracts the positive proton more strongly, so each removal is harder. Phosphorus stays at +5 throughout, so no oxidation state change is involved.

Question 12

A weak monobasic acid is 2% ionised in a 0.16 M solution. The solution is diluted to 0.01 M at constant temperature. Assuming α is small compared with 1, what are the new degree of ionisation and the change in [H⁺]?
  1. α stays at 0.02 and [H⁺] falls from 3.2 × 10⁻³ M to 2 × 10⁻⁴ M
  2. α becomes 0.08 and [H⁺] falls from 3.2 × 10⁻³ M to 8 × 10⁻⁴ M
  3. α becomes 0.08 and [H⁺] rises from 3.2 × 10⁻³ M to 1.28 × 10⁻² M
  4. α becomes 0.32 and [H⁺] stays at 3.2 × 10⁻³ M
Show answer
Answer: (B) α becomes 0.08 and [H⁺] falls from 3.2 × 10⁻³ M to 8 × 10⁻⁴ M
Explanation: By Ostwald's dilution law, Ka = Cα², so Ka = 0.16 × (0.02)² = 6.4 × 10⁻⁵. At 0.01 M, α = √(6.4 × 10⁻⁵/0.01) = 0.08, four times larger. Then [H⁺] = Cα = 0.01 × 0.08 = 8 × 10⁻⁴ M, which is lower than the original 0.16 × 0.02 = 3.2 × 10⁻³ M. Dilution raises the fraction ionised but lowers the actual H⁺ concentration.

Question 13

Weak monobasic acids HX (Ka = 8.0 × 10⁻⁵) and HY (Ka = 2.0 × 10⁻⁵) are each present as 0.10 M solutions. Compared with the HY solution, the pH of the HX solution is (log 2 = 0.30)
  1. higher by about 0.60
  2. higher by about 0.30
  3. lower by about 0.30
  4. lower by about 0.60
Show answer
Answer: (C) lower by about 0.30
Explanation: For a weak acid at concentration C, [H⁺] = √(KaC), so at equal concentration the ratio of [H⁺] is √(8.0 × 10⁻⁵ / 2.0 × 10⁻⁵) = √4 = 2. HX therefore has twice the [H⁺] of HY. Its pH is lower by log 2 = 0.30.

Question 14

The acid strength in water follows the order HCl > CH₃COOH > H₂O > C₂H₅OH. What is the correct order of decreasing basic strength of their conjugate bases?
  1. CH₃COO⁻ > Cl⁻ > C₂H₅O⁻ > OH⁻
  2. C₂H₅O⁻ > OH⁻ > CH₃COO⁻ > Cl⁻
  3. OH⁻ > C₂H₅O⁻ > Cl⁻ > CH₃COO⁻
  4. Cl⁻ > CH₃COO⁻ > OH⁻ > C₂H₅O⁻
Show answer
Answer: (B) C₂H₅O⁻ > OH⁻ > CH₃COO⁻ > Cl⁻
Explanation: The stronger an acid, the weaker its conjugate base, because a strong acid has little tendency to take its proton back. HCl is the strongest acid here, so Cl⁻ is the weakest base. Ethanol is the weakest acid, so the ethoxide ion is the strongest base, and basic strength runs opposite to the given acid order.

Question 15

Consider three reactions: (i) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l); (ii) NH₃(g) + HCl(g) → NH₄Cl(s); (iii) BF₃ + NH₃ → F₃B–NH₃. Which option gives the narrowest of the Arrhenius, Brønsted–Lowry and Lewis concepts that still covers each reaction?
  1. (i) Arrhenius; (ii) Lewis; (iii) Brønsted–Lowry
  2. (i) Lewis; (ii) Brønsted–Lowry; (iii) Arrhenius
  3. (i) Arrhenius; (ii) Brønsted–Lowry; (iii) Lewis
  4. (i) Brønsted–Lowry; (ii) Arrhenius; (iii) Lewis
Show answer
Answer: (C) (i) Arrhenius; (ii) Brønsted–Lowry; (iii) Lewis
Explanation: Reaction (i) is H⁺ combining with OH⁻ in water, which the Arrhenius concept covers. Reaction (ii) has a proton transfer but occurs in the gas phase with no aqueous H⁺ or OH⁻, so Brønsted–Lowry is needed. Reaction (iii) involves no proton at all; it is an electron pair donated by NH₃ to BF₃, so only the Lewis concept applies.

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