Free Acids, Bases and Salts MCQs with Answers

261. Which of the following statements is correct?

• A. H3PO3 is dibasic and reducing
• B. H3PO3 is tribasic and reducing
• C. H3PO3 is tribasic and non-reducing
• D. H3PO3 is dibasic and non-reducing

Explanation: In the structure of H3PO3, two -OH groups are present, so it is dibasic acid. In it one P-H bond is present, so it provides hydrogen and due to such hydrogen it acts as a reducing agent.

262. What is the correct relation between pH and pKa?

• A. pH = pKa + log[ Acid/Base ]
• B. pH = pKa - log[ Acid/Base ]
• C. pH = pKa - log[ Base/Acid ]
• D. pH = pKa + log[ Base]

Explanation: The Henderson-Hasselbalch equation relates pKa and pH. However, it is only an approximation and should not be used for concentrated solutions or for extremely low pH acids or high pH bases.pH = pKa + log ([conjugate base]/[weak acid])pH = pka + log ([A-] / [HA])pH is equal to the sum of the pKa value and the log of the conjugate base concentration divided by the weak acid concentration.

263. The conjugated acid of NH3 is:

• A. NH4+
• B. NH
• C. NH2
• D. NH2-
• E. NH3

Explanation: NH₄⁺: This is the conjugated acid of NH₃ (ammonia). When NH₃ accepts a proton (H⁺), it forms NH₄⁺.

264. The strongest conjugate base is:

• A. NO3-
• B. Cl-
• C. SO42-
• D. CH3COO-

Explanation: The strength of a conjugate base is inversely related to the strength of its conjugate acid. A weaker acid results in a stronger conjugate base. CH3COO- is the conjugate base of acetic acid, which is a weak acid; hence, it is the strongest conjugate base among the options. NO3- (from HNO3), Cl- (from HCl), and SO42- (from H2SO4) are derived from stronger acids, making them weaker conjugate bases.

265. An acid is a substance which accepts:

• A. An electron pair
• B. A proton
• C. An electron
• D. Pair of protons

Explanation: The correct answer is that an acid is a substance which accepts an electron pair, according to the Lewis concept. This concept broadens the traditional definition of acids beyond the Bronsted-Lowry theory, which focuses on proton donation. Thus, Option A is correct. Option B is incorrect as it describes the action of acids in the Bronsted-Lowry concept, which involves donating, not accepting, a proton. Option C is incorrect because it misinterprets the Lewis concept, emphasizing that acids accept electron pairs instead of single electrons. Option D is incorrect as no accepted theory or concept describes acids as accepting pairs of protons; the focus remains on single proton donation in the Bronsted-Lowry framework.

266. Which of the following is Lewis acid?

• A. FeCl₃
• B. AlCl₃
• C. BF3
• D. All of these

Explanation: The correct answer is All of these because all the compounds listed (FeCl₃, AlCl₃, and BF3) are Lewis acids. A Lewis acid is defined as a substance that can accept a pair of electrons to form a covalent bond. In FeCl₃, the iron atom is electron-deficient and can accept an electron pair. Similarly, AlCl₃ has an electron-deficient aluminum atom that can accept an electron pair, and BF3 has a boron atom with an incomplete octet, making it capable of accepting an electron pair. Therefore, each of these compounds can act as a Lewis acid.

267. Which of the following is strong base?

• A. NH₃
• B. PH₃
• C. RNH₂
• D. R₂NH

Explanation: The correct answer is R₂NH because secondary amines (R₂NH) are generally stronger bases than primary amines (RNH₂) and ammonia (NH₃). This is due to the presence of additional alkyl groups that donate electron density to the nitrogen atom, enhancing its ability to donate a lone pair of electrons. In contrast, NH₃ is a weak base as it does not completely ionize in solution. PH₃ is even weaker than NH₃ because phosphorus is less electronegative than nitrogen and forms weaker hydrogen bonds. Primary amines, RNH₂, are intermediate in strength, as they have only one alkyl group to donate electron density.

268. The pH of milk is

• A. 5.0
• B. 6.0
• C. 6.2
• D. 6.5

Explanation: The correct pH of milk is around 6.5, indicating it is slightly acidic. The pH scale ranges from 0 to 14, with values below 7 indicating acidity. Milk's slight acidity is due to the presence of lactic acid and other naturally occurring compounds. Option A (5.0) represents a more acidic solution than milk. Option B (6.0) and Option C (6.2) are closer but still slightly more acidic than typical milk. Option D (6.5) correctly reflects the average pH of milk, making it the best answer.

269. Which of the following ions can act both as bronsted acid and base in solvent water?

• A. CN-
• B. SO4-2
• C. CHO3-
• D. PO4-3

Explanation: CHO3- can act as a Bronsted acid and a Bronsted base. A Bronsted acid is a species that donates a proton (H+), while a Bronsted base is a species that accepts a proton(H+). - In the presence of a stronger base, such as OH-, the H+ ion in CHO3- can be donated to form H2O and CO32-. This reaction makes CHO3- act as a Bronsted acid. - In the presence of a stronger acid, such as HCl, CHO3- can accept an H+ ion to form H2CO3. This reaction makes CHO3- act as a Bronsted base.

270. An acid is a substance which accepts:

• A. An electron pair
• B. Proton
• C. An electron
• D. Pair of proton

Explanation: By Bronsted-Lowery concecpt an acid is a substance which lose a proton. By Lewis concept an acid is a substance which accepts An electron pair.