📚🌟 Model Test – Acids, Bases & Salts
⚗️🧮 XI Chemistry
✍️🧮 Short Answer Questions 🎯📘
✍️🧮 Descriptive Questions 🎯📘
🔢 📖 Numerical Problems 🧮✨
📘✨ Textbook MCQs on Acids, Bases and Salts
Q1. H₂SO₄ is stronger acid than CH₃COOH because:
✅ Answer: (c)
Explanation: Acid strength depends on degree of ionization and Ka. H₂SO₄ has ~60% ionization and Ka ≈ 10³, making it very strong. CH₃COOH has only ~1.4% ionization and Ka ≈ 1.7×10⁻⁵, so it is weaker.
Q2. Which statement is not correct about bases?
✅ Answer: (d)
Explanation: Bases turn red litmus paper blue, not blue litmus red.
Q3. Al₂O₃ is amphoteric oxide because it reacts with:
✅ Answer: (c)
Explanation: Amphoteric oxides react with both acids and bases to form salt and water. Examples: Al₂O₃, ZnO, BeO, SnO₂, PbO₂, Cr₂O₃.
Q4. Which of the following is NOT a buffer solution?
✅ Answer: (d)
Explanation: Buffer = weak acid + salt of strong base OR weak base + salt of strong acid. NaOH/HCl are both strong, so not a buffer.
Q5. Which oxide is amphoteric in nature?
✅ Answer: (d)
Explanation: Al₂O₃ reacts with both acids and bases. Basic oxides: K₂O, Na₂O, CaO. Acidic oxides: CO₂, SO₂, P₂O₅. Neutral oxides: CO, NO.
Q6. Which of the following does not alter the pH of a solution?
✅ Answer: (c)
Explanation: Neutral salts formed from strong acids and strong bases like NaCl, KBr, KNO₃ etc. do not change the pH of solution.
Q7. Conjugate acid of NH₃ is:
✅ Answer: (a)
Explanation: Adding H⁺ to NH₃ gives NH₄⁺ (conjugate acid). Removing H gives NH₂⁻ (conjugate base).
Q8. Salt formed by neutralization of weak acid and strong base is:
✅ Answer: (c)
Explanation: Na₂CO₃ is formed from weak acid H₂CO₃ and strong base NaOH.
Q9. A conjugate acid-base pair differs by only:
✅ Answer: (b)
Explanation: Conjugate acid-base pairs differ by a single proton (H⁺). Example: NH₃/NH₄⁺, HCl/Cl⁻.
Q10. Salt formed by neutralization of weak acid and weak base is:
✅ Answer: (c)
Explanation: NH₄CN is formed from weak acid HCN and weak base NH₄OH.
📘✨ Newly Compiled Textbook MCQs on Acids, Bases and Salts
Q1. The aqueous solution of KCl is:
✅ Answer: (a)
Explanation: KCl is a neutral salt formed from strong acid (HCl) and strong base (KOH).
Q2. Which one of the following yields a basic solution?
✅ Answer: (c)
Explanation: Sodium acetate is a salt of weak acid (CH₃COOH) and strong base (NaOH), giving basic solution.
Q3. Universal indicator turns red in:
✅ Answer: (c)
Explanation: Universal indicator shows red in strongly acidic solutions (pH < 4).
Q4. Which one of the following is NOT a strong acid?
✅ Answer: (a)
Explanation: HF is a weak acid compared to strong acids like HCl, HNO₃, and H₂SO₄.
Q5. Which one of the following substances has pH value more than 7?
✅ Answer: (b)
Explanation: Sodium carbonate is a basic salt, so its aqueous solution has pH > 7.
Q6. Which one of the following is NOT a salt of weak acids and strong bases?
✅ Answer: (c)
Explanation: Ammonium chloride is a salt of weak base (NH₄OH) and strong acid (HCl).
Q7. Which one of the following is NOT a salt of strong acids and weak bases?
✅ Answer: (c)
Explanation: Sodium acetate is a salt of weak acid (CH₃COOH) and strong base (NaOH).
Q8. Which one of the following salts dissolves in water to produce a solution with pH less than 7?
✅ Answer: (d)
Explanation: None of these salts produce an acidic solution (pH < 7) when dissolved in water.
Q9. Which one of the following salts dissolves in water to produce a solution with pH more than 7?
✅ Answer: (d)
Explanation: None of these salts produce a basic solution (pH > 7) when dissolved in water.
Q10. Which of the following salts has no water of crystallization?
✅ Answer: (c)
Explanation: Baking soda (NaHCO₃) does not contain water of crystallization, unlike gypsum or washing soda.
Q11. Which ion is not present in dilute sulfuric acid?
✅ Answer: (c)
Explanation: Dilute H₂SO₄ contains H⁺ and SO₄²⁻ ions, not SO₃²⁻.
Q12. Which salt is NOT derived from a strong acid and a strong soluble base?
✅ Answer: (a)
Explanation: MgCl₂ comes from strong acid HCl but weakly soluble base Mg(OH)₂.
Q13. The aqueous solution of NaCl is:
✅ Answer: (a)
Explanation: NaCl is a neutral salt formed from strong acid (HCl) and strong base (NaOH).
Q14. Which one of the following yields a basic solution?
✅ Answer: (c)
Explanation: KCN is a salt of weak acid (HCN) and strong base (KOH), giving basic solution.
Q15. Universal indicator turns green in:
✅ Answer: (a)
Explanation: Universal indicator shows green in neutral solutions (pH ≈ 7).
Q16. NH₃ can be a base according to:
✅ Answer: (d)
Explanation: NH₃ acts as a base in Bronsted-Lowry (proton acceptor) and Lewis (electron donor) theories.
Q17. Which of the following is a Lewis base?
✅ Answer: (b)
Explanation: CN⁻ can donate an electron pair, so it is a Lewis base.
Q18. A substance that can donate a pair of electrons to form coordinate covalent bond:
✅ Answer: (d)
Explanation: Lewis bases donate electron pairs to form coordinate covalent bonds.
Q19. If pH value is greater than 7, then solution is:
✅ Answer: (b)
Explanation: Solutions with pH > 7 are basic (alkaline).
Q20. Which of the following is an acid-base conjugate pair?
✅ Answer: (c)
Explanation: H₂CO₃ and CO₃²⁻ differ by one proton, making them a conjugate acid-base pair.
Q21. What will be the pH of aqueous solution of NH₄Cl?
✅ Answer: (d)
Explanation: NH₄Cl is a salt of weak base (NH₄OH) and strong acid (HCl), so its solution is acidic (pH < 7).
Q22. Which one of the following is non-hydrolyzable salt?
✅ Answer: (b)
Explanation: NaCl is a neutral salt of strong acid (HCl) and strong base (NaOH), hence non-hydrolyzable.
Q23. Aqueous solution of NaCl has a pH:
✅ Answer: (a)
Explanation: NaCl solution is neutral with pH = 7.
Q24. Hydrolysis of potassium acetate gives:
✅ Answer: (a)
Explanation: Potassium acetate is a salt of weak acid (CH₃COOH) and strong base (KOH), giving basic solution.
Q25. Aqueous solution of CuSO₄ has a pH:
✅ Answer: (b)
Explanation: CuSO₄ solution is acidic due to hydrolysis of Cu²⁺ ions, so pH < 7.
Q26. Which of the following phenomena will occur when a small amount of acid is added to water?
✅ Answer: (d)
Explanation: Adding acid to water causes dilution and ionization of the acid molecules.
Q27. Strong bases are:
✅ Answer: (b)
Explanation: Strong bases like NaOH and KOH ionize completely in water, acting as strong electrolytes.
Q28. Whose definition of acids and bases emphasizes the role of protons?
✅ Answer: (c)
Explanation: Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors.
Q29. An electron-pair acceptor is a:
✅ Answer: (c)
Explanation: Lewis acids accept electron pairs, while Lewis bases donate them.
Q30. Which one of the given is hydracid?
✅ Answer: (b)
Explanation: Hydracids are acids composed of hydrogen and a non-metal; HCN is a hydracid.
Q31. Which element is always present in Arrhenius acid?
✅ Answer: (b)
Explanation: Arrhenius acids always contain hydrogen, which they release as H⁺ ions in solution.
Q32. Ammonium sulphate salt is:
✅ Answer: (c)
Explanation: Ammonium sulphate is formed from weak base (NH₄OH) and strong acid (H₂SO₄), so it is acidic.
Q33. Which of the following does not form an acidic salt?
✅ Answer: (b)
Explanation: HCl is a monoprotic strong acid, so it cannot form acidic salts.
Q34. A Brønsted-Lowry base is defined as:
✅ Answer: (c)
Explanation: Brønsted-Lowry bases are defined as proton (H⁺) acceptors.
Q35. The number of water molecules attached to the ionic compounds depends upon………. of the ion.
✅ Answer: (c)
Explanation: The extent of hydration depends on the charge density of the ion; higher charge density attracts more water molecules.
Q36. Which one of the following ions is readily hydrated?
✅ Answer: (a)
Explanation: Al³⁺ has high charge density, so it is strongly hydrated compared to Mg²⁺ or Na⁺.
Q37. Which of the following is readily hydrated?
✅ Answer: (a)
Explanation: Cations attract water molecules more strongly due to their positive charge.
Q38. The number of water molecules attached to the ionic compounds depends upon………. of the ion.
✅ Answer: (c)
Explanation: Hydration depends on both charge and ionic size; higher charge density increases hydration.
Q39. A salt X is dissolved in water of pH 7, the resulting solution becomes alkaline in nature. Salt X is made up of:
✅ Answer: (a)
Explanation: Salts of weak acids and strong bases hydrolyze to give alkaline solutions.
Q40. A salt X is dissolved in water of pH 7, the resulting solution becomes acidic in nature. Salt X is made up of:
✅ Answer: (d)
Explanation: Salts of strong acids and weak bases hydrolyze to give acidic solutions.
Q41. Which of the given is a strong base?
✅ Answer: (d)
Explanation: Potassium hydroxide (KOH) is a strong base because it ionizes completely in aqueous solution.
Q42. An indicator is one kind of the following compound:
✅ Answer: (c)
Explanation: Indicators are weak acids or bases that change color depending on the pH of the solution.
Q1. Define pH and pOH of a solution. Also show that pH + pOH = 14.
💧 pH (Power of Hydrogen → power of H⁺ ion concentration)
Defined as: the negative logarithm of the molar hydrogen ions concentration [H⁺] (in mol/litre) of an aqueous solution at a given temperature.
Formula: pH = −log [H⁺] = log 1/[H⁺] AND [H⁺] = antilog (−pH)
Acidity Indicator: Measures acidity of solution.
Range: 0–14.
✨ Extra Note:
➡️Low pH (0–6) → acidic 🍋
➡️pH = 7 → neutral 💧
➡️High pH (8–14) → basic 🧼
🌊 pOH (Power of Hydroxide → power of OH⁻ ion concentration)
Defined as: the negative logarithm of molar concentration of OH− ion in aqueous solution at a given temperature.
pOH = −log [OH⁻] = log 1/[OH⁻] AND [OH⁻] = antilog (−pOH)
basicity Indicator: Measures basicity of solution.
Range: 0–14.
🎯 Relation Between pH & pOH (pH + pOH = 14 (neutral water at 25°C)
From water dissociation constant:
Kꙍ = [H⁺][OH⁻] = 1×10⁻¹⁴
Taking negative log:
(−log Kꙍ = (−log [H⁺]) (−log [OH⁻])
−log (1 x 10⁻¹⁴) = (−log [H⁺]) (−log [OH⁻])
14 = pH + pOH or pH + pOH = 14 (at 25°C)
Q2. Why the aqueous solution of NH₄Cl is acidic and Na₂CO₃ is alkaline.
🔎 🟥 Hydrolysis of Salts of Strong Acid and Weak Base (Acidic Salts → Cationic Hydrolysis)
🧪 Formed by strong acid + weak base.
⚡ Anion (from strong acid) → no hydrolysis.
💧 Cation (from weak base) → reacts with water → releases H⁺ (Cationic Hydrolysis).
📉 pH < 7 → solution becomes acidic.
✨ Examples: NH₄Cl, (NH₄)₂SO₄, CuCl₂.
🎯 Quick Memory Cue: 👉 Strong acid + weak base → cation hydrolyzes → acidic solution (pH < 7).
🟥 NH₄Cl → Acidic Solution (Cationic Hydrolysis)
➡️Formed from strong acid (HCl) + weak base (NH₄OH).
➡️Cl⁻ (anion from strong acid) → no hydrolysis.
➡️NH₄⁺ (cation from weak base) → strong conjugate acid → hydrolyzes with water → releases H⁺ ions → pH < 7 → acidic
➡️NH₄Cl ⇌ NH₄⁺ + Cl⁻ (Ionization)
➡️NH₄⁺ + H–OH → NH₄OH + H⁺ (Cationic Hydrolysis)
🔎 🟦 Hydrolysis of Salts of Weak Acid and Strong Base (Basic Salts → Anionic Hydrolysis)
🧪 Formed by weak acid + strong base.
⚡ Anion (from weak acid) → reacts with water → releases OH⁻ (Anionic Hydrolysis).
💧 Cation (from strong base) → no hydrolysis.
📉 pH > 7 → solution becomes basic.
✨ Examples: Na₂CO₃, CH₃COONa, NaHCO₃ etc.
🎯 Quick Memory Cue: 👉 weak acid + Strong base → anion hydrolyzes → Basic solution (pH > 7).
🟦 Na₂CO₃ → Alkaline Solution (Anionic Hydrolysis)
➡️Formed from weak acid (H₂CO₃) + strong base (NaOH).
➡️Na⁺ (cation from strong base) → no hydrolysis.
➡️CO₃²⁻ (anion from weak acid) → strong conjugate base → hydrolyzes with water → releases OH⁻ ions → pH > 7 → basic solution
➡️Na₂CO₃ ⇌ 2Na⁺ + CO₃²⁻ (Ionization)
➡️CO₃²⁻ + 2H–OH → H₂CO₃ + 2OH⁻
🎯 Quick Memory Cue
🟥 NH₄Cl → cationic hydrolysis → acidic (pH < 7).
🟦 Na₂CO₃ → anionic hydrolysis → alkaline (pH > 7).
Q3. Write down conjugate base of each of the following acids: H₂SO₄, H₂S, NH₄⁺, HCOOH
🧪 Acid (Species) ➖ Remove H⁺ 🎯 Conjugate Base
H₂SO₄ − H⁺ → HSO₄⁻
H₂S − H⁺ → HS⁻
NH₄⁺ − H⁺ → NH₃
HCOOH − H⁺ → HCOO⁻
🎯 Quick Memory Cue
Conjugate base = acid minus one proton (H⁺).
Conjugate acid = base add one proton (H⁺).
Stronger acid → weaker conjugate base ⚖️.
Weaker acid → stronger conjugate base 💪.
Alternate Smart Style
🔢 Conjugate Bases of Given Acids
➡️H₂SO₄ → remove one proton (H⁺) → HSO₄⁻
➡️H₂S → remove one proton (H⁺) → HS⁻
➡️NH₄⁺ → remove one proton (H⁺) → NH₃
➡️HCOOH → remove one proton (H⁺) → HCOO⁻
🎯 Quick Memory Cue
Conjugate base = acid minus one proton (H⁺).
Conjugate acid = base add one proton (H⁺).
Stronger acid → weaker conjugate base ⚖️.
Weaker acid → stronger conjugate base 💪.
Q4. What is meant by self-ionization of water? Write the expression of Kw. What is its value at 25°C?
💧Water Electrolytic Nature
Water is mostly non electrolyte but conducts slightly due to auto ionization.
One H₂O donates a proton to another → forms H₃O⁺ and OH⁻.
Self Ionization of Water Equation: H₂O + H₂O ⇌ H₃O⁺ + OH⁻
Basis of Expression
💧 Water ionizes slightly → H₃O⁺ + OH⁻
🔄 Equilibrium established characterized by an equilibrium constant (Kc) at a given temperature
📉 Unionized water in large excess ≈ constant
⚡ Kw = ionic product of water → equilibrium constant for water auto-ionization equilibrium → very small value
⚡ Expression of Kw (Ionic Product of Water/ Ionic-Product constant/ Dissociation or ionization constant)
Kc = [H₃O⁺][OH⁻]/[H₂O][H₂O]
Kc = [H₃O⁺][OH⁻]/[H₂O]² ([H₂O]² = constant)
Kc·[H₂O]² = [H₃O⁺][OH⁻]
Kꙍ = [H₃O⁺][OH⁻] (Kc·[H₂O]² = Kꙍ = constant)
Kw = product of molar concentrations of H₃O⁺ (hydrogen ion) and OH⁻ (hydroxide ion) at a particular temperature (at 25°C).
At 25°C, Kw = 1×10⁻¹⁴ mol²/dm⁶
🎯 Quick Memory Cue
💧 Self ionization → H₂O ⇌ H₃O⁺ + OH⁻
⚡ Kw = [H₃O⁺][OH⁻]
📉 Value at 25°C = 1×10⁻¹⁴
Q5. Write equation and indicated the conjugate acid-base pairs for the following:
(i) Acetic acid and ammonia (ii) Hydrochloric acid and water
CH₃COOH + NH₃(g) ⇌ NH₄⁺(aq) + CH₃COO⁻(aq)
(Acid) (Base) (Conjugate acid) (Conjugate base)
HCl(aq) + H₂O ⇌ H₃O⁺(aq) + Cl⁻(aq)
(Acid) (Base) (Conjugate acid) (Conjugate base)
Q6. Define and explain acidic, basic and amphoteric oxides with example.
Metal Oxides
Metal oxides are basic in nature and they are readily dissolves in water to produced respective soluble metal hydroxide producing alkaline solution.
e.g. Na₂O, K₂O, CaO etc.
Na₂O(s) + H₂O(l) → 2NaOH(aq)
K₂O(s) + H₂O(l) → 2KOH(aq)
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Non-metal Oxides
Non-metal oxides are acidic in nature and they are readily dissolves in water to produced respective oxyacid producing acidic solution.
e.g. CO₂, SO₂, P₂O₅ etc.
CO₂(g) + H₂O(l) → H₂CO₃(aq)
SO₂(g) + H₂O(l) → H₂SO₃(aq)
P₂O₅(g) + H₂O(l) → 2H₃PO₄(aq)
Amphoteric Oxides
Certain amphoteric metal oxides which are insoluble in water have ability to react with both acid and base and are borderline of both acid and base are called amphoteric oxides or amphoteric substances.
e.g. Al₂O₃, ZnO etc.
Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂ + H₂O(l)
🌊 Strength by degree of dissociation (α) / % ionization
**Definition:** α = fraction of molecules ionized; % ionization = α × 100.
**Rule of thumb:** Higher % ionization → stronger acid/base (strong ≳30%, weak ≲30%).
| Acid | % ionization | Base | % ionization |
|---|---|---|---|
| HCl | 90–95% | NaOH | 90–95% |
| H₂SO₄ (1st) | ~60% | Ba(OH)₂ (1st) | ~77% |
| CH₃COOH | ~1.4% | NH₄OH | ~1.4% |
📈 Strength by ionization constants (Kₐ, Kь)
**Principle:** Larger Kₐ → stronger acid; larger Kь → stronger base (Bronsted–Lowry).
**Equilibria:**
• Acid: HA + H₂O ⇌ H₃O⁺ + A⁻ → Kₐ = [H₃O⁺][A⁻]/[HA]
• Base: B + H₂O ⇌ BH⁺ + OH⁻ → Kь = [BH⁺][OH⁻]/[B]
| Weak acid | Kₐ | Weak base | Kь |
|---|---|---|---|
| Oxalic (H₂C₂O₄) | 5.6×10⁻² | Ethylamine (C₂H₅NH₂) | 4.7×10⁻⁴ |
| Acetic (CH₃COOH) | 1.7×10⁻⁵ | Ammonia (NH₃) | 1.8×10⁻⁵ |
| Carbonic (H₂CO₃) | 4.3×10⁻⁷ | Pyridine (C₅H₅N) | 1.4×10⁻⁹ |
🌟 Direct takeaway: High % ionization or high Kₐ/K_b values signal stronger acids/bases; low values indicate weak ones.
💧 Levelling Effect:
The influence of solvent on the apparent strength of acids and bases is called the levelling effect. In water, strong acids (HCl, HBr, HI) all appear equally strong because they ionize completely, producing H₃O⁺ — the strongest acid possible in aqueous medium.
⚡ For Acids:
Any acid stronger than H₃O⁺ is levelled to the same strength, since:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻
⚡ For Bases:
Any base stronger than OH⁻ is levelled to OH⁻, since:
NH₂⁻ + H₂O ⇌ NH₃ + OH⁻
🌟 Key Point: In aqueous solutions, H₃O⁺ is the strongest acid and OH⁻ is the strongest base that can exist. Stronger species are “levelled” down to these limits by water.
🧪 Applications of Some Salts:
Salts are widely used in industry and daily life. Examples are shown below:
| Common Name | Chemical Name | Formula | Applications |
|---|---|---|---|
| Table salt | Sodium chloride | NaCl | Essential part of human diet |
| Epsom salt | Magnesium sulphate | MgSO₄ | Laxative agent |
| Gypsum salt | Calcium sulphate | CaSO₄ | Making Plaster of Paris |
| Baking soda | Sodium bicarbonate | NaHCO₃ | Antacid for indigestion |
| Washing soda | Sodium carbonate | Na₂CO₃ | Laundry stain remover |
| Sal ammoniac | Ammonium chloride | NH₄Cl | Expectorant in cough syrup |
| Muriate of potash | Potassium chloride | KCl | Fertilizer production |
📊 Classification based on [H⁺] compared to 1×10⁻⁷ mol·dm⁻³:
| Case | [H⁺] (mol·dm⁻³) | Classification |
|---|---|---|
| (i) | 1×10⁻⁴ | Acidic |
| (ii) | 1×10⁻¹¹ | Basic |
| (iii) | 1×10⁻⁹ | Basic |
| (iv) | 1×10⁻³ | Acidic |
🔁 Conjugate Base: Formed by removing a proton (H⁺) from the acid.
| Species | Conjugate Base |
|---|---|
| HS⁻ | S²⁻ |
| H₃O⁺ | H₂O |
| H₂PO₄⁻ | HPO₄²⁻ |
| HSO₄⁻ | SO₄²⁻ |
| HF | F⁻ |
| CH₃COOH | CH₃COO⁻ |
| [Al(H₂O)₆]³⁺ | [Al(H₂O)₅OH]²⁺ |
🔁 Conjugate Acid: Formed by adding a proton (H⁺) to the base.
| Species | Conjugate Acid |
|---|---|
| OH⁻ | H₂O |
| HCO₃⁻ | H₂CO₃ |
| HPO₄²⁻ | H₂PO₄⁻ |
| CH₃NH₂ | CH₃NH₃⁺ |
| CO₃²⁻ | HCO₃⁻ |
| CH₃COOH | CH₃COOH₂⁺ |
🔁 Dual Behavior: A species that can both donate and accept a proton acts as both Brønsted acid and base.
🧪 Species with dual behavior:
• H₂O → can donate H⁺ (acid) or accept H⁺ (base)
• HS⁻ → can lose H⁺ to become S²⁻ or gain H⁺ to become H₂S
• HCO₃⁻ → can lose H⁺ to become CO₃²⁻ or gain H⁺ to become H₂CO₃
🌟 Conclusion: H₂O, HS⁻, HCO₃⁻ behave both as Brønsted acids and Brønsted bases.
(i) What type of salt will you have?
➡️ Mixing a strong acid and strong base results in complete neutralization, forming a normal salt (NaNO₃).
(ii) What type of reaction will it be?
➡️ This is a neutralization reaction — acid + base → salt + water.
(iii) Will it be a soluble or insoluble salt?
➡️ Nitrate salts (like NaNO₃) are always water-soluble.
(iv) If it is soluble, how will it be recovered?
➡️ Soluble salts are recovered by crystallization — evaporating water to obtain pure salt crystals.
(i) Why pure water is not a strong electrolyte?
➡️ Pure water ionizes only slightly into H⁺ and OH⁻ ions due to strong hydrogen bonding, making it a weak electrolyte.
(ii) Why ionic-product constant of water is temperature dependent?
➡️ The ionic product constant (Kw) increases with temperature because higher temperature enhances dissociation of H₂O into H⁺ and OH⁻ ions.
(iii) Which kind of bond is formed between Lewis acid and a base? Name the product formed.
➡️ Lewis acid–base reactions form a coordinate covalent bond, producing an adduct.
Example: H₃N: + BF₃ → H₃N→BF₃ (Lewis adduct)
(iv) Why H⁺ ion acts as a Lewis acid?
➡️ The proton (H⁺) is electron-deficient and readily accepts a lone pair of electrons, behaving as a Lewis acid.
Example: H₃N: + H⁺ → H₄N⁺ (Lewis adduct)
📘 Answers of Descriptive Questions
📖 Brønsted–Lowry Concept (1923):
• Acid: Proton donor (H⁺)
• Base: Proton acceptor (H⁺)
➡️ Acid–base reactions involve transfer of a proton from acid to base.
🧪 Examples of Acids: HCl, HNO₃, H₂SO₄, H₂O, H₃O⁺, NH₄⁺
🧪 Examples of Bases: NH₃, H₂O, OH⁻, Cl⁻, CO₃²⁻, HSO₄⁻
🔁 Conjugate Acid–Base Pair:
• When an acid loses a proton, it forms its conjugate base.
• When a base gains a proton, it forms its conjugate acid.
📌 Example Reaction:
HCl(aq) + H₂O ⇌ H₃O⁺(aq) + Cl⁻(aq)
• HCl → acid, Cl⁻ → conjugate base
• H₂O → base, H₃O⁺ → conjugate acid
🌟 Key Point: Conjugate pairs differ only by one proton (H⁺).
🔁 Conjugate Acid–Base Pair:
A conjugate acid–base pair consists of two species that differ by a single proton (H⁺).
• Acid → donates a proton → forms its conjugate base
• Base → accepts a proton → forms its conjugate acid
🧪 Example 1: Reaction between H₂S and NH₃
H₂S + NH₃ ⇌ NH₄⁺ + HS⁻
• H₂S → acid → HS⁻ (conjugate base)
• NH₃ → base → NH₄⁺ (conjugate acid)
🧪 Example 2: Dissolution of acetic acid in water
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
• CH₃COOH → acid → CH₃COO⁻ (conjugate base)
• H₂O → base → H₃O⁺ (conjugate acid)
🌟 Key Point: Conjugate pairs are always linked by the transfer of a single proton.
(i) K₂CO₃ (ii) (NH₄)₂SO₄ (iii) NaNO₃
💧 Hydrolysis: A proton-transfer reaction where ions of a salt react with water, splitting it into H⁺ and OH⁻, forming acidic or basic solutions depending on the nature of the salt.
🔹 (i) K₂CO₃ → Salt of Weak Acid & Strong Base
• Type: Anionic hydrolysis
• Reaction:
CO₃²⁻(aq) + 2H₂O(l) ⟶ H₂CO₃(aq) + 2OH⁻(aq)
• Result: Basic solution (pH > 7)
🔹 (ii) (NH₄)₂SO₄ → Salt of Strong Acid & Weak Base
• Type: Cationic hydrolysis
• Reaction:
NH₄⁺(aq) + H₂O(l) ⟶ NH₄OH(aq) + H⁺(aq)
• Result: Acidic solution (pH < 7)
🔹 (iii) NaNO₃ → Salt of Strong Acid & Strong Base
• Type: No hydrolysis (only hydration)
• Reaction:
NaNO₃(aq) ⇌ Na⁺(aq) + NO₃⁻(aq)
• Result: Neutral solution (pH = 7)
🌟 Summary:
• K₂CO₃ → basic
• (NH₄)₂SO₄ → acidic
• NaNO₃ → neutral
💧 Buffer Solution: A solution that resists changes in pH upon dilution or addition of small amounts of acid or base. It contains a weak acid and its conjugate base (acidic buffer) or a weak base and its conjugate acid (basic buffer).
🔁 Buffer Action: The ability of a buffer to maintain pH by neutralizing added H⁺ or OH⁻ ions.
⚖️ Buffer Capacity: The amount of acid or base a buffer can neutralize before pH changes significantly. It is highest when acid/base and salt concentrations are equal.
🧪 Mechanism (Acetic Acid–Sodium Acetate Buffer):
•
CH₃COOH ⇌ CH₃COO⁻ + H⁺
CH₃COONa ⇌ CH₃COO⁻ + Na⁺
• On adding acid (HCl):
H⁺ + CH₃COO⁻ ⇌ CH₃COOH → pH remains stable
• On adding base (NaOH):
OH⁻ + CH₃COOH ⇌ H₂O + CH₃COO⁻ → pH remains stable
📌 Applications: Buffers are used in biological systems, pharmaceuticals, fermentation, and analytical chemistry to maintain stable pH.
📘 Lewis Theory (1923):
• Lewis Acid: Electron pair acceptor
• Lewis Base: Electron pair donor
➡️ Lewis acid–base reactions form a coordinate covalent bond and produce a single compound called a Lewis adduct.
🧪 General Reaction:
A (acid) + •B (base) ⟶ A←B or A—B
• A = electron pair acceptor (Lewis acid)
• B = electron pair donor (Lewis base)
🧪 Example:
NH₃ + AlCl₃ ⟶ NH₃→AlCl₃ (Lewis adduct)
• NH₃ = Lewis base (donates lone pair)
• AlCl₃ = Lewis acid (accepts lone pair)
🧪 Examples of Lewis Acids:
1. Molecules with incomplete octet: BF₃, BCl₃, FeCl₃, B(OH)₃, AlCl₃, AlBr₃, ZnCl₂
2. Molecules with polar multiple bonds: CO₂, SO₂, SO₃
3. Molecules with vacant d-orbitals: SF₄, SF₆, SnCl₂, SnCl₄
4. Metal cations and H⁺: Li⁺, Ag⁺, Al³⁺, Mg²⁺, H⁺
🧪 Examples of Lewis Bases:
1. Neutral molecules with lone pairs: NH₃, PH₃, H₂O, R–NH₂, R₂NH, ROR
2. Anions with lone pairs or π-bonds: O²⁻, SO₄²⁻, CO₃²⁻, Cl⁻, Br⁻, I⁻, CH₃COO⁻, OH⁻
🔍 Advantages over Brønsted–Lowry Theory:
• Explains acid–base behavior without requiring H⁺ transfer
• Applies to non-protonic reactions (e.g. metal–ligand bonding)
• Covers broader range of chemical species (e.g. BF₃, AlCl₃, CO₂)
🌟 Conclusion:
Lewis theory expands the concept of acids and bases beyond proton transfer, making it more universal and applicable to coordination chemistry.
📘 Definition:
A salt is an ionic compound produced by the neutralization of an acid and a base, consisting of the positive ion from the base and the negative ion from the acid.
🔹 Types of Salts (based on neutralization):
| Acid | Base | Type of Salt | Reaction |
|---|---|---|---|
| Strong | Strong | Neutral Salt | NaOH + HCl ⟶ NaCl + H₂O |
| Strong | Weak | Acidic Salt | NH₄OH + HCl ⟶ NH₄Cl + H₂O |
| Weak | Strong | Basic Salt | NaOH + CH₃COOH ⟶ CH₃COONa + H₂O |
| Weak | Weak | Neutral Salt | NH₄OH + CH₃COOH ⟶ CH₃COONH₄ + H₂O |
🌟 Summary:
• Neutral salts: formed by strong acid + strong base (e.g. NaCl)
• Acidic salts: formed by strong acid + weak base (e.g. NH₄Cl)
• Basic salts: formed by weak acid + strong base (e.g. CH₃COONa)
• Neutral salts (weak–weak): formed by weak acid + weak base (e.g. CH₃COONH₄)
📘 Solutions of Numerical Problems (pH, pOH, [H⁺], [OH⁻])
Molarity of HCl solution = mass/molar mass × Vdm³ = 14.8 / 36.5 × 0.75 = 0.54 M
(HCl being a strong acid, it is completely ionized in aqueous solution. For every HCl molecule, one H⁺ ion is produced.)
[H⁺] = [HCl] = 0.54 M = 5.4 × 10⁻¹ M
⚖️ Ionization Equation:
HCl ⇌ H⁺(aq) + Cl⁻(aq)
0.54 M → 0.54 M
📌 pH Calculation:
pH = – log [H⁺]
= – log (5.4 × 10⁻¹)
= – [log 5.4 + log 10⁻¹]
= – [0.732 – 1]
= – (–0.267)
= 0.267
🌟 Answer: pH ≈ 0.27
⚖️ Kw Expression:
[H⁺][OH⁻] = 1 × 10⁻¹⁴
[H⁺] × 3.5 × 10⁻⁴ = 1 × 10⁻¹⁴
[H⁺] = (1 × 10⁻¹⁴) / (3.5 × 10⁻⁴) = 2.86 × 10⁻¹¹ M
📌 pH Calculation:
pH = – log [H⁺]
= – log (2.86 × 10⁻¹¹)
= – [log 2.86 + log 10⁻¹¹]
= – [0.456 – 11]
= – (–10.54)
= 10.54
🌟 Answer: pH ≈ 10.54
📌 pOH Calculation:
pH + pOH = 14
pOH = 14 – 2.3 = 11.7
📌 [H⁺] Calculation:
[H⁺] = 10⁻ᵖᴴ = 10⁻²·³ = 5.01 × 10⁻³ M
🌟 Answer: pOH = 11.7, [H⁺] ≈ 5.01 × 10⁻³ M
⚖️ Ionization Equation:
KOH ⇌ K⁺ + OH⁻
2.46 × 10⁻⁹ M → 2.46 × 10⁻⁹ M
📌 [OH⁻] Calculation:
[OH⁻] = [KOH] = 2.46 × 10⁻⁹ M
📌 [H⁺] Calculation:
[H⁺][OH⁻] = Kw = 1 × 10⁻¹⁴
[H⁺] = (1 × 10⁻¹⁴) / (2.46 × 10⁻⁹) = 4.07 × 10⁻⁶ M
📌 pH Calculation:
pH = – log [H⁺] = – log (4.07 × 10⁻⁶) = 5.39
📌 pOH Calculation:
pOH = 14 – pH = 14 – 5.39 = 8.61
🌟 Answer: pH = 5.39, pOH = 8.61, [OH⁻] = 2.46 × 10⁻⁹ M, [H⁺] = 4.07 × 10⁻⁶ M
🔹 Definition of Oxides:
Oxides are binary compounds of oxygen with other elements such as metals, non-metals, or metalloids.
🔹 Definition of Salts:
A salt is an ionic crystalline compound formed by the neutralization of an acid and a base. It consists of a cation (from base, other than H⁺) and an anion (from acid, other than OH⁻). Example: NaCl, CuCl₂.
🔹 Types of Oxides Based on Acid–Base Properties:
| Type | Definition | Equations | Examples |
|---|---|---|---|
| Metal Oxides / Basic Oxides | Dissolve in water to produce soluble hydroxides (basic solution). |
Na₂O(s) + H₂O(l) → 2NaOH(aq) K₂O(s) + H₂O(l) → 2KOH(aq) CaO(s) + H₂O(l) → Ca(OH)₂(aq) |
Na₂O, K₂O, CaO |
| Non-metal Oxides / Acidic Oxides | Dissolve in water to produce oxyacids (acidic solution). |
CO₂(g) + H₂O(l) → H₂CO₃(aq) SO₂(g) + H₂O(l) → H₂SO₃(aq) P₂O₅(g) + H₂O(l) → 2H₃PO₄(aq) |
CO₂, SO₂, P₂O₅ |
| Amphoteric Oxides | Insoluble in water but react with both acids and bases. |
Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂ + H₂O(l) Al₂O₃(s) + 6HCl(aq) → 2AlCl₃ + 3H₂O(l) |
Al₂O₃, ZnO |
🌟 Summary:
• Metal oxides → basic solutions
• Non-metal oxides → acidic solutions
• Amphoteric oxides → react with both acids and bases
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