Free Acids, Bases and Salts MCQs with Answers

91. Which of the following is not the use of a buffer solution?

• A. Use for calibration of pH meter
• B. Use to preserve biological specimen
• C. Maintain pH of human blood
• D. Predict the concentration of a substance

Explanation: Buffer solutions are not used to predict the concentration of a substance. They are primarily used to maintain a stable pH. However, buffer solutions can be employed in certain analytical techniques, such as spectrophotometry, to ensure the accurate measurement of a substance's concentration by maintaining the desired pH conditions. Nonetheless, the buffer solution itself does not directly predict the concentration of the substance being analyzed.

92. A solution which does not tend to vary in pH is called?

• A. Acidic solution
• B. Buffer solution
• C. Bases solution
• D. Super saturated solution

Explanation: A solution that does not tend to vary in pH is called a buffer solution. Therefore, option B is correct.

93. If weak acid is diluted with water, then H+ ions concentration will

• A. decrease
• B. gradually decreases then increase
• C. increase
• D. remain same

Explanation: When a weak acid is diluted with water, the concentration of hydrogen ions [𝐻+] will decrease. This is because dilution reduces the overall concentration of all species in the solution, including the hydrogen ions produced by the dissociation of the weak acid.

94. Choose which one of the following can not be classed as buffer solution:

• A. KH2PO4/H3PO4
• B. NaClO4/HClO4
• C. CH3COOH/CH3COONa
• D. NH4OH/NH4Cl

Explanation: The correct answer is Option B: NaClO4/HClO4. A buffer solution consists of a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. NaClO4 and HClO4 do not form a buffer because they are products of a strong acid and a strong base, which do not resist pH changes effectively when an acid or base is added. In contrast, the other options contain either a weak acid and its conjugate base or a weak base and its conjugate acid, which are able to maintain a stable pH.

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

• A. CN-
• B. SO42-
• C. HCO3-
• D. PO43-

Explanation: The correct answer is HCO3- because it can function as both a Bronsted acid and a Bronsted base in water. As a Bronsted acid, it donates a proton (H+) to form CO32-. As a Bronsted base, it accepts a proton to form H2CO3. The other options do not have this dual capability:CN-: Functions mainly as a Bronsted base by accepting protons.SO42-: Predominantly acts as a Bronsted base and is the conjugate base of a strong acid.PO43-: Primarily acts as a Bronsted base by accepting protons.

96. The proton acceptor is:

• A. NH3
• B. BF3
• C. HCl
• D. H+

Explanation: According to the Bronsted-Lowry theory, a base is a substance that accepts protons. NH3 is a classic example of a weak base that acts as a proton acceptor. In water, it reacts to form NH4+ and OH- ions, demonstrating its ability to accept a proton. In contrast, BF3 is a Lewis acid that accepts electron pairs, not protons. HCl is a proton donor, releasing H+ ions in water, and is therefore classified as an acid. Lastly, H+ is already a proton, so it cannot act as a proton acceptor.

97. The pH of 0.001M aqueous solution of NaOH is:

• A. 6
• B. 13
• C. 11
• D. 12

Explanation: To find the pH of a 0.001M NaOH solution, first recognize that NaOH is a strong base and fully dissociates in water. This means the concentration of OH- ions is 0.001M, or 10-3M. The pOH is calculated as -log(10-3) = 3. Since pH + pOH = 14, the pH is 14 - 3 = 11, confirming that the correct answer is 11. Other options are incorrect because they do not properly apply the relationship between pH and pOH or misinterpret the concentration of the solution.

98. The aqueous solution of which one of the following compounds maintain its pH constant?

• A. CH3COOH and (NH4)2SO4
• B. NH4NO3 and KNO3
• C. NH4OH and NH4Cl
• D. NH4OH and NaCl

Explanation: A buffer solution is designed to maintain a stable pH when small amounts of an acid or base are added. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. In this question, the correct answer is NH4OH and NH4Cl. NH4OH is a weak base, and NH4Cl provides the conjugate acid (NH4+), forming a buffer system that resists changes in pH. The other options do not meet the criteria for forming effective buffer solutions: CH3COOH and (NH4)2SO4 do not form a compatible acid-base pair; NH4NO3 and KNO3 are salts that do not buffer; and NH4OH and NaCl lack the necessary acidic component.

99. According to the Bronsted Lowery concept, which of the following species cannot function as an acid?

• A. SO42-
• B. H3O-
• C. HSO4-
• D. NH4+

Explanation: According to the Brønsted-Lowry concept, an acid is a species that can donate a proton (H+). The sulfate ion, SO42-, cannot donate a proton because it does not have any protons to donate, making it incapable of acting as an acid. In contrast, H3O-, HSO4-, and NH4+ all have protons that they can potentially donate, which allows them to function as acids according to this definition.

100. Compounds that tend to donate electron pairs are known as Lewis acid or nucleophiles. Which one of the following is not a Lewis acid?

• A. CH3-NH2
• B. PH3
• C. AlCl3
• D. H2O

Explanation: A Lewis base is a substance that can donate an electron pair. Option C, AlCl3, is the correct answer as it acts as a Lewis acid due to its ability to accept electron pairs to complete its octet. In contrast, CH3-NH2, PH3, and H2O all have lone pairs of electrons that can be donated, classifying them as Lewis bases.