Free Acids, Bases and Salts MCQs with Answers

151. Which of the following will give a buffer solution with pH more than 7?

• A. Weak acid & its salt with a strong base
• B. Weak acid & its salt with a weak base
• C. Weak base & its salt with a strong acid
• D. Weak base & its salt with a weak base

Explanation: The correct answer is Option C: 'Weak base & its salt with a strong acid'. A buffer solution with a pH greater than 7 is known as a basic buffer. Basic buffers are typically composed of a weak base and its salt with a strong acid. The weak base partially neutralizes the added strong acid, maintaining the solution's pH above 7. Options A and B involve weak acids, which do not contribute to a basic buffer, while Option D lacks the acid component necessary for buffering.

152. If pK_(a) of value of acid is 6.4, then what should be the ratio between salt to acid concentration to prepared a buffer solution of pH = 5.4?

• A. 1 : 1
• B. 1 : 10
• C. 10 : 1
• D. 1 : 2

Explanation: To determine the ratio of salt to acid concentration for the buffer solution, apply the Henderson-Hasselbalch equation: pH = pKa + log([Salt]/[Acid]). Given pH = 5.4 and pKa = 6.4, subtracting gives: 5.4 = 6.4 + log([Salt]/[Acid]) -1 = log([Salt]/[Acid]). Solving for the ratio: [Salt]/[Acid] = 10-1 = 1/10, or 1:10. Therefore, the correct ratio is 1:10, which supports the buffer solution at a pH of 5.4. The other options either suggest equal concentrations or incorrect ratios that would not achieve the desired pH.

153. Which of the following is having high pH buffer?

• A. H3PO4/ NaH2PO4
• B. H2CO3/NaHCO3
• C. NH4OH/(NH4)2SO4
• D. CH3COOH/CH3COONa

Explanation: The correct answer is NH4OH/(NH4)2SO4 because ammonium hydroxide is a weak base, which contributes to a higher pH buffer system. In contrast, the other options involve acid or mildly acidic buffer systems. H3PO4/NaH2PO4 and CH3COOH/CH3COONa are acidic, while H2CO3/NaHCO3 is neutral.

154. Which of the following type of salt can act as a buffer?

• A. Salt of strong acid & strong base
• B. Salt of strong acid & weak base
• C. Salt of weak acid & weak base
• D. All of the above

Explanation: A buffer solution is typically composed of a weak acid and its conjugate base or a weak base and its conjugate acid. Therefore, a salt formed from a strong acid and a weak base can act as a buffer because it can supply the conjugate acid needed to maintain pH stability. In contrast, salts of strong acids and strong bases do not resist changes in pH, and salts of weak acids and weak bases do not effectively maintain a stable pH.

155. Which of the following govern the action of buffer solution?

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

Explanation: The correct answer is the Leveling effect. This effect describes how the strength of a strong acid or base is 'leveled' to the strength of the acid or base characteristic of the solvent, which is crucial in the behavior of buffer solutions. Buffer solutions resist changes in pH because the leveling effect ensures that the acids or bases added to the solution are neutralized to some extent.Le-Chatelier's principle applies to systems at equilibrium and is not specifically related to the action of buffer solutions.The Solvent effect is more about how solvents influence solute properties, which is not directly governing buffer actions.None of these is incorrect because the leveling effect indeed plays a key role in the function of buffers.

156. Sodium benzoate and benzoic acid of equimolar mixed to form buffer if pKa is 2 what will be pH?

• A. 1
• B. 2
• C. 0
• D. None of these

Explanation: The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). In this question, sodium benzoate acts as the conjugate base ([A-]) and benzoic acid is the weak acid ([HA]). Since they are mixed in equimolar amounts, [A-] = [HA], thus the log term becomes zero. Therefore, the pH equals the pKa, which is 2. Option B is correct. Option A and C are incorrect as they do not match the pKa value, and Option D is incorrect because there is indeed a correct answer.

157. The capability of a buffer solution to resist the change in pH in particular range of pH is called

• A. Acidic capacity
• B. Basic capacity
• C. Buffer capacity
• D. None of these

Explanation: The correct answer is Buffer capacity. Buffer capacity is a measure of the efficiency of a buffer in resisting changes in pH upon the addition of an acid or a base. It is a property of a buffer solution that allows it to maintain a relatively constant pH. The terms acidic capacity and basic capacity refer to the ability of a solution to neutralize added acids or bases, respectively, but they do not describe the overall pH stability that buffer capacity provides. None of these is incorrect because the correct answer is listed among the options.

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

• A. 1 mole of NH3 with 2 moles of HCl
• B. 0.0002 mole of HCl
• C. 2 moles of NaCl with 2 mole of HCl
• D. 4 moles of CH3COOH with 4 mole of NaCl

Explanation: A buffer solution is typically made from a weak acid and its conjugate base, or a weak base and its conjugate acid. Option D is the correct answer because it includes acetic acid (CH3COOH), a weak acid, and the sodium ion from NaCl. The CH3COO- ion acts as the conjugate base, forming an effective buffer. Options A, B, and C do not have the necessary components to form a buffer system: Option A is an acidic solution, Option B is a dilute acid, and Option C is a combination of a strong acid and a salt, none of which can resist pH changes effectively.

159. Addition of CH3COONa in water gives

• A. Standard solution
• B. Buffer solution
• C. Basic solution
• D. Acidic solution

Explanation: When CH3COONa (sodium acetate) dissolves in water, it dissociates into CH3COO- and Na+. The acetate ion (CH3COO-) undergoes hydrolysis with water, forming acetic acid (CH3COOH) and hydroxide ions (OH-), thus increasing the concentration of OH- and making the solution basic. Therefore, the correct answer is 'Basic solution.' The other options are incorrect: 'Standard solution' and 'Buffer solution' do not describe the nature of the solution formed, and 'Acidic solution' is incorrect because the formation of OH- results in a basic solution.

160. Buffer solution can be prepared by mixing

• A. Strong acid & strong base
• B. Strong base & its salt
• C. Weak base & its conjugate salt
• D. Weak acid & weak base

Explanation: A buffer solution is composed of a weak acid with its conjugate base or a weak base with its conjugate acid, allowing it to resist changes in pH upon the addition of small amounts of acid or base. In the case of a weak base and its conjugate salt, the weak base partially dissociates, and the salt provides the necessary conjugate acid for buffering. Strong acids and bases fully dissociate and neutralize each other, which does not provide the necessary conditions for a buffer. A weak acid and a weak base lack the conjugate pairs needed to maintain pH stability, thus failing to form a buffer solution.