Free Acids, Bases and Salts MCQs with Answers

121. Base is acceptor of

• A. OH⁻ ion
• B. Proton
• C. Electron pair
• D. All of these

Explanation: In the context of the Bronsted-Lowry theory, a base is defined as a substance that accepts protons (H⁺ ions). Therefore, the correct answer is Option B: Proton. Option A is incorrect because OH⁻ ions are not accepted by bases; instead, bases can produce OH⁻ ions in aqueous solutions. Option C is incorrect as it refers to the Lewis definition of bases, which involves donating electron pairs, not accepting protons. Option D is incorrect because a base does not accept all of these, as it specifically accepts protons according to the Bronsted-Lowry definition.

122. Conjugate base of HCO₃⁻ is

• A. H₂CO₃
• B. CO₃²⁻
• C. All of these
• D. None of the above

Explanation: The conjugate base of an acid is formed when the acid loses a proton (H⁺). Here, the bicarbonate ion, HCO₃⁻, loses a proton to become CO₃²⁻, the carbonate ion, making CO₃²⁻ the correct conjugate base. Option A, H₂CO₃, is incorrect because it represents the conjugate acid of HCO₃⁻, which forms when a proton is added. Option C, All of these, is incorrect because only CO₃²⁻ is the conjugate base of HCO₃⁻. Option D, None of the above, is incorrect since CO₃²⁻ is the right answer.

123. Which of the following is strong base?

• A. NaOH
• B. H₂O
• C. H₃O⁺
• D. NH₃

Explanation: The correct answer is NaOH. Sodium hydroxide is a strong base because it completely dissociates in water to release hydroxide ions (OH⁻), resulting in a high pH level. H₂O is neutral and does not dissociate to release significant amounts of OH⁻. H₃O⁺ is actually a hydronium ion, which is an indicator of an acidic solution, not basic. NH₃ (Ammonia) is a weak base that only partially dissociates in water, making it less effective than strong bases like NaOH.

124. Which of the following base is strong among the following?

• A. NO₃⁻ (aq)
• B. Cl⁻ (aq)
• C. CH₃COO⁻ (aq)
• D. HSO₄⁻ (aq)

Explanation: The strength of a base can be inferred from the strength of its conjugate acid. A strong acid has a weak conjugate base, and a weak acid has a relatively stronger conjugate base. Among the given options, CH₃COO⁻ is the conjugate base of acetic acid, which is a weak acid. This makes CH₃COO⁻ a relatively stronger base compared to the others. NO₃⁻ and Cl⁻ are conjugate bases of strong acids (HNO₃ and HCl, respectively), making them weak bases. HSO₄⁻, while being a conjugate base of a strong acid (H₂SO₄), can still donate a proton, so it is not a strong base.

125. Which of the following is strong acid?

• A. I₂CHCOOH
• B. F₂CHCOOH
• C. Br₂CHCOOH
• D. Cl₂CHCOOH

Explanation: The strength of an acid is influenced by the electronegativity of atoms attached to the carboxylic acid group. Fluorine is the most electronegative halogen, which increases the acidity of F₂CHCOOH due to its greater ability to stabilize the negative charge on the conjugate base. In contrast, iodine is the least electronegative, resulting in a weaker acid. Bromine and chlorine fall between fluorine and iodine in terms of electronegativity, making their corresponding acids of moderate strength.

126. Conjugate acid of a very strong base is relatively ...............

• A. Strong acid
• B. Very weak acid
• C. Neutral
• D. Both a & b

Explanation: The strength of a base and its conjugate acid are inversely related. When a base is very strong, it easily accepts protons, meaning its conjugate acid does not easily donate protons, making it a very weak acid. Option B is correct because it aligns with this concept. A strong acid (Option A) is incorrect because a very strong base cannot form a strong conjugate acid. Neutral (Option C) is also incorrect since conjugate acids/bases are not inherently neutral. Option D is incorrect because it suggests a contradiction: a substance cannot be both a strong and weak acid simultaneously.

127. Why is OH⁻ ion a strong base in water solution?

• A. OH⁻ is only the base in the solution
• B. OH⁻ is the conjugate base of H₂O
• C. Strong bases dissolve completely in water and produce OH⁻ ions
• D. All bases react with water to produce OH⁻ ions

Explanation: OH⁻ is considered a strong base in water because it is the conjugate base of water, H₂O. When water loses a proton, it forms OH⁻. As a strong base, OH⁻ can readily accept protons. Option B is correct because it correctly identifies OH⁻ as the conjugate base of H₂O. Option A is incorrect as it oversimplifies the role of OH⁻. Option C is misleading because it does not address why OH⁻ is inherently a strong base, and Option D is incorrect because not all bases follow this behavior.

128. The acid having most stable conjugate base in aqueous solution among the following is:

• A. HI
• B. HNO3
• C. H2SO4
• D. HCN

Explanation: The correct answer is HI. In aqueous solution, HI dissociates to generate the iodide ion (I-), which is highly stable due to its large size and the ability to distribute its negative charge effectively. This makes the conjugate base of HI very stable. On the other hand, HNO3 forms the nitrate ion (NO3-), which, while stable due to resonance, is less stable than iodide. H2SO4 results in the bisulfate ion (HSO4-), which has partial charge delocalization but is less stable than iodide. HCN forms the cyanide ion (CN-), which is smaller and less stable as a conjugate base compared to iodide.

129. An acid is not strong if its conjugate base:

• A. Have higher pKb value
• B. Highly stable
• C. Have lower Ka value
• D. None of the above

Explanation: The strength of an acid is inversely related to the strength of its conjugate base. If an acid has a lower Ka value, it is weaker, meaning it dissociates less in solution. Consequently, its conjugate base is relatively stronger in accepting protons. Therefore, Option C is correct. Option A is incorrect because a higher pKb suggests a weaker base, which would mean the original acid is stronger, not weaker. Option B is incorrect because a highly stable conjugate base usually indicates that the acid is strong. Option D is incorrect because Option C is the correct choice.

130. The pH scale is introduced by

• A. Thomson
• B. Sørensen
• C. Graham
• D. Henderson

Explanation: The correct answer is Sørensen, as he introduced the concept of the pH scale in 1909. The pH scale is a measure of the acidity or alkalinity of a solution. Other options are associated with different scientific contributions: Thomson with the electron, Graham with gas diffusion, and Henderson with buffer solutions.