Free Acids, Bases and Salts MCQs with Answers

181. Which of the following is strongest acid among these?

• A. Benzoic acid
• B. Phenol
• C. Acetic acid
• D. Water

Explanation: Benzoic acid is the strongest acid among the options provided because the benzene ring allows for resonance stabilization of the carboxylate ion after deprotonation, enhancing its acidity. Phenol, although it has some acidity due to the hydroxyl group, does not benefit from the same degree of resonance stabilization. Acetic acid, while a carboxylic acid, lacks the aromatic ring that contributes to the increased acidity in benzoic acid. Water is neutral and lacks significant acidity, as it does not contain any acidic functional groups comparable to the other options.

182. Pka value of phenol is

• A. 7.9
• B. 8.9
• C. 9.9
• D. 10.9

Explanation: Phenol is a weak acid with a pKa of approximately 8.9, which reflects its ability to donate a proton. The pKa value is crucial in understanding the relative acidity of a compound; the lower the pKa, the stronger the acid. Option B correctly identifies the pKa of phenol as 8.9. Option A is incorrect because it suggests phenol is a stronger acid than it is. Options C and D are incorrect because they suggest phenol is weaker than it actually is, with increasing pKa values indicating decreasing acidity.

183. Which one is weakest base among these?

• A. Aniline
• B. Pyridine
• C. Ammonia
• D. Methylamine

Explanation: Aniline is the weakest base among the options because the nitrogen's lone pair of electrons is delocalized into the benzene ring, making it less available for protonation. In contrast, pyridine, ammonia, and methylamine have nitrogen atoms with lone pairs that are more available for bonding with protons. Pyridine's nitrogen is part of an aromatic heterocycle, but its lone pair is not involved in resonance, making it more basic than aniline. Ammonia has a straightforward structure with an available lone pair, and methylamine's nitrogen is further activated by the electron-donating effect of the methyl group, making them both stronger bases.

184. The aqueous solution of which of the following can form buffer solution?

• A. HF & NaF
• B. NH₃ & NH₄Cl
• C. CO₃²⁻ & Na₂CO₃
• D. All of these

Explanation: A buffer solution is formed by a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. In option A, HF (weak acid) and NaF (conjugate base) can form a buffer. Option B, NH₃ (weak base) and NH₄Cl (conjugate acid) also form a buffer. Similarly, in option C, CO₃²⁻ and Na₂CO₃ form a buffer system with CO₃²⁻ acting as the base and HCO₃⁻ (from Na₂CO₃) as the acid. Therefore, option D, 'All of these,' correctly indicates that all pairs can form buffer solutions.

185. Which of the following have almost same pH?

• A. Blood & egg (white)
• B. Tears & blood
• C. Tears & milk
• D. Milk & egg (white)

Explanation: The correct answer is Option B: Tears & blood. Both tears and blood have a pH close to 7.4, which is slightly alkaline. This makes them much more similar in pH compared to the other options. Blood and egg white (Option A) are somewhat close, but not as close as tears and blood. Tears and milk (Option C) have different pH values, with milk being more acidic. Similarly, milk and egg white (Option D) also differ significantly in pH, with egg white being more alkaline.

186. Which of the following form acidic buffer?

• A. Acetic acid / sodium acetate
• B. Phosphoric acid / potassium dihydrogen phosphate
• C. Formic acid / sodium formate
• D. All of these

Explanation: An acidic buffer solution is typically composed of a weak acid and its conjugate base (in the form of a salt). Each pair in the options includes a weak acid and its corresponding salt: acetic acid with sodium acetate, phosphoric acid with potassium dihydrogen phosphate, and formic acid with sodium formate. Therefore, all these combinations can form acidic buffers. Selecting 'All of these' as the correct answer is appropriate because every pair listed fulfills the criteria for forming an acidic buffer.

187. Salt of weak acid & weak base is

• A. NH₄NO₃
• B. NaCN
• C. NH₄NO₂
• D. K₂CO₃

Explanation: The correct answer is NH₄NO₂. This salt is formed from the weak acid HNO₂ and the weak base NH₃, which means it fulfills the criteria of being derived from both a weak acid and a weak base. NH₄NO₃ is incorrect because it is derived from a weak base (NH₃) and a strong acid (HNO₃). NaCN is incorrect because it is derived from a strong base (NaOH) and a weak acid (HCN). K₂CO₃ is incorrect because it is derived from a strong base (KOH) and a weak acid (H₂CO₃). To identify the nature of a salt, consider the strengths of the acid and base that form it.

188. In case of acidic buffer solution the concentration of conjugate base is prominently supplied by

• A. Acid
• B. Salt
• C. Base
• D. Solvent

Explanation: An acidic buffer solution is composed of a weak acid and its conjugate base, often supplied by a salt of the acid. This salt dissociates in solution to provide the conjugate base, which is crucial for buffering capacity. For example, in a solution of acetic acid and sodium acetate, the sodium acetate dissociates to supply acetate ions (the conjugate base). This helps the buffer resist changes in pH when small amounts of acid or base are added.The other options are incorrect because:Acid: While an acid is part of the buffer, it provides hydrogen ions rather than the conjugate base.Base: A base is not typically used to provide the conjugate base in an acidic buffer system.Solvent: The solvent does not contribute to the buffer's ability to neutralize added acids or bases.

189. "Acids are substances whose aqueous solutions turned blue litmus red and tasted sour " stated by

• A. Davy
• B. Liebig
• C. Boyle
• D. Rouelle

Explanation: The correct answer is Robert Boyle. Boyle is known for his early work in chemistry, where he described the properties of acids, including their sour taste and their ability to turn blue litmus paper red. This definition is fundamental to the understanding of acids in chemistry. The other scientists, while significant in their own right, did not define acids in this manner. Davy focused on electrochemistry, Liebig contributed significantly to organic chemistry, and Rouelle had different contributions unrelated to the litmus test for acids.

190. Arrhenius concept explained

• A. Constant heat of neutralization
• B. Quantitative determination of acid/base strength
• C. Catalytic property of acid
• D. The dissociation of acids and bases in water

Explanation: The Arrhenius concept defines acids as substances that increase the concentration of H+ ions in aqueous solutions, while bases increase the concentration of OH- ions. It does not directly address the constant heat of neutralization, the quantitative determination of acid/base strength, or the catalytic properties of acids, which are unrelated to the fundamental definitions in the Arrhenius concept.