Free Acids, Bases and Salts MCQs with Answers

31. A solution of Na₂SO₄ is:

• A. Basic
• B. Acidic
• C. Neutral
• D. Not predictable

Explanation: Sodium sulfate (Na₂SO₄) is a salt formed from a strong acid (H₂SO₄) and a strong base (NaOH). Neither of its ions will hydrolyze water, so the resulting solution is neutral.

32. Which of the following is a mixed salt?

• A. KAl(SO₄)₂
• B. Na₂CO₃
• C. Ca(OCl)Cl
• D. NaCl

Explanation: A mixed salt is a salt that contains more than one type of cation or anion. Bleaching powder, Ca(OCl)Cl, contains both the hypochlorite (OCl⁻) and chloride (Cl⁻) anions, making it a mixed salt.

33. The aqueous solution of sodium phosphate (Na₃PO₄) is:

• A. Acidic
• B. Basic
• C. Neutral
• D. Amphoteric

Explanation: Sodium phosphate is the salt of a strong base (NaOH) and a weak acid (HPO₄²⁻). The phosphate ion (PO₄³⁻) is a strong conjugate base and will hydrolyze water to produce OH⁻ ions, making the solution basic.

34. The strength of strong bases can be distinguished in a:

• A. Protogenic solvent
• B. Protophilic solvent
• C. Aprotic solvent
• D. Organic solvent

Explanation: A protogenic (acidic) solvent can differentiate the strengths of strong bases. A very strong base will react more completely with the acidic solvent than a moderately strong base, allowing their relative strengths to be observed. This is known as the differentiating effect.

35. Which of the following is the Henderson-Hasselbalch equation?

• A. pH = pKb + Log([Salt]/[Acid])
• B. pH = pKa + Log([Salt]/[Acid])
• C. pH = pKb + Log([Acid]/[Salt])
• D. pH = pKa + Log([Acid]/[Salt])

Explanation: This is the correct form of the Henderson-Hasselbalch equation used for calculating the pH of an acidic buffer. It relates the pH to the pKa of the weak acid and the ratio of the concentrations of the conjugate base (salt) and the acid.

36. Which of the following governs the action of a buffer solution?

• A. Leveling effect
• B. Solvent effect
• C. Le Chatelier's principle
• D. None of these

Explanation: The action of a buffer is an excellent example of Le Chatelier's principle. When a strong acid or base is added, the buffer's equilibrium shifts to consume the added H⁺ or OH⁻, thereby resisting a large change in pH.

37. Which set of solutes will form a buffer when dissolved in water to make 1 liter of solution?

• A. 4 moles of NH₃ with 2 moles of HCl
• B. 0.0002 mole of HCl
• C. 2 moles of NaCl with 2 moles of HCl
• D. 4 moles of CH₃COOH with 4 moles of NaCl

Explanation: NH₃ is a weak base, and partial neutralization with HCl produces NH₄⁺ (its conjugate acid). After reaction, both NH₃ and NH₄⁺ are present → a classic buffer pair. So only Option A forms a buffer.

38. Which of the following can form a buffer solution?

• A. HNO₂ and NaNO₂
• B. HCN and NaCN
• C. NH₃ and (NH₄)₂SO₄
• D. All of these

Explanation: All of the options represent a mixture of a weak acid with its conjugate base (A and B) or a weak base with its conjugate acid (C). Therefore, all three combinations can be used to prepare buffer solutions.

39. Which of the following oxides is amphoteric in character?

• A. CaO
• B. CO₂
• C. SiO₂
• D. SnO₂

Explanation: Amphoteric oxides react with both acids and bases. Among the options, SnO₂ (tin oxide) shows this dual behavior. The others are either acidic or basic only.

40. The aqueous solution of which of the following salts is basic?

• A. NH₄Cl
• B. NaCl
• C. CH₃COONa
• D. NaNO₃

Explanation: Sodium acetate (CH₃COONa) is the salt of a weak acid (CH₃COOH) and a strong base (NaOH). The acetate ion (CH₃COO⁻) hydrolyzes water to produce OH⁻ ions, making the solution basic.