Free Acids, Bases and Salts MCQs with Answers

131. pKw =14 at 25 °C, the pKw at 100 °C is

• A. pKw = 14
• B. pKw > 14
• C. pKw < 14
• D. None of these

Explanation: The ionization constant of water (Kw) increases as temperature increases, which means more water molecules ionize into hydrogen and hydroxide ions at higher temperatures. Since pKw is the negative logarithm of Kw, an increase in Kw results in a decrease in pKw. At 25 °C, pKw is 14, but at 100 °C, it is less than 14. Thus, the correct answer is pKw < 14. The other options suggest either no change or incorrect changes in the ionization of water with temperature, which do not align with the known behavior of water.

132. What is the pH of 0.001M NaOH solution?

• A. 3
• B. 7
• C. 11
• D. 10

Explanation: To calculate the pH of a 0.001M NaOH solution, first determine its pOH. Since NaOH is a strong base and dissociates completely, the concentration of OH⁻ ions is 0.001M. The pOH is calculated as -log(0.001) = 3. Since pH + pOH = 14 for aqueous solutions, the pH is 14 - 3 = 11. Therefore, the correct answer is Option C: 11. Option A: 3 suggests an acidic solution, Option B: 7 indicates neutrality, and Option D: 10 underestimates the basicity of the solution.

133. The solution having zero pH will be

• A. Neutral
• B. Basic
• C. Highly acidic
• D. Highly basic

Explanation: The pH scale ranges from 0 to 14 and is used to determine the acidity or basicity of a solution. A pH of 7 is considered neutral, a pH less than 7 is acidic, and a pH greater than 7 is basic. A pH of zero indicates an extremely high concentration of hydrogen ions, making the solution highly acidic. Therefore, the solution with zero pH is highly acidic. The other options are incorrect because neutral solutions have a pH of 7, basic solutions have pH values greater than 7, and highly basic solutions have pH values closer to 14.

134. Which of the following solution have zero pH

• A. 1M HCl
• B. 1M HF
• C. 1M HNO3
• D. 1M CH3COOH

Explanation: The correct answer is 1M HCl. Hydrochloric acid (HCl) is a strong acid that completely dissociates in water, resulting in a high concentration of hydrogen ions (H+), which can cause the pH to approach zero. Other options like 1M HF and 1M CH3COOH are weak acids and do not fully dissociate, leading to higher pH levels. While 1M HNO3 is a strong acid and also has a low pH, its pH is typically higher than zero but very close. Therefore, 1M HCl is the most suitable option for having a pH of zero.

135. The pH of a solution is 5, the hydroxyl ion concentration in mol dm3 is

• A. 10-5
• B. 109
• C. 105.5
• D. 10-7

Explanation: To find the hydroxyl ion concentration, we first determine the pOH from the given pH. The relationship is pH + pOH = 14. With a pH of 5, the pOH is 14 - 5 = 9. The hydroxyl ion concentration is then calculated as 10 to the power of negative pOH, which is 10-9 mol dm3. Option A represents the hydrogen ion concentration, not hydroxyl ion concentration. Option C does not fit the calculation. Option D represents a neutral condition and is not relevant to this acidic solution.

136. When small amount of acid is added to water at 25 °C then the Kw

• A. Increases
• B. Decreases
• C. Remain constant
• D. May increase or decrease

Explanation: The ion product of water (Kw) is a constant at a given temperature, specifically 25 °C in this case. When an acid is added to water, the concentrations of H+ and OH- ions change, but the product of these concentrations, Kw, remains constant. This is because Kw is dependent solely on temperature and not on the presence of acids or bases. Therefore, the correct answer is that Kw remains constant. Options suggesting that Kw increases, decreases, or may vary are incorrect because they do not recognize the temperature-dependent nature of Kw.

137. Which of the following is correct about 1.0 x 10^-3 M HCl solution & 1.0 x 10^5 M HClO4 solution

• A. HCl solution is less acidic than HClO4 solution
• B. HCl solution is 100 times more acidic than HClO4 solution
• C. HClO4 is 100 times more acidic than HCl solution
• D. Not predictable

Explanation: The acidity of a solution is determined by the concentration of hydrogen ions (H+). The concentration of H+ ions from 1.0 × 10-3 M HCl is higher than that from 1.0 × 105 M HClO4. Therefore, the HCl solution is 100 times more acidic than the HClO4 solution. The other options are incorrect as they misinterpret the concentration values or suggest unpredictability, which is not the case here.

138. When CO2 gas is dissolved in water, the pH of the water.

• A. Increases
• B. Decreases
• C. Remain constant
• D. May increase or decrease

Explanation: When CO2 gas is dissolved in water, it reacts to form carbonic acid (H2CO3), a weak acid. This reaction increases the hydrogen ion concentration in the solution, thereby lowering the pH and making the solution more acidic. Therefore, the correct answer is that the pH decreases (Option B). Option A is incorrect because the pH does not increase; a more basic solution would result from a decrease in hydrogen ions. Option C is incorrect because the addition of CO2 does affect the pH. Option D is incorrect because the pH does not vary unpredictably; it decreases specifically due to the formation of carbonic acid.

139. pH of a solution, which has H+ ion concentration of 0.0002 mol dm-3

• A. 3.0
• B. 3.69
• C. 4.5
• D. 2.69

Explanation: The pH of a solution is calculated by taking the negative logarithm (base 10) of the hydrogen ion concentration. In this case, the concentration of H+ ions is 0.0002 mol dm-3. Using the formula pH = -log[0.0002], we find the pH to be approximately 3.69. Option A (3.0) and Option D (2.69) are incorrect due to calculation errors, and Option C (4.5) is too high for the given concentration.

140. A piece of paper is burnt in air, the gas produced is passed through distilled water, the pH of water will be

• A. 7
• B. 3
• C. 6.8
• D. 2.1

Explanation: When paper, which is composed of organic material, is burnt, carbon dioxide (CO2) is produced. When CO2 is passed through distilled water, it reacts to form carbonic acid (H2CO3), a weak acid. This reaction slightly lowers the pH of the water from 7 to around 6.8, making Option C the correct choice. Option A is incorrect because it assumes no change in pH, which does not account for the acidic nature of carbonic acid. Option B assumes a stronger acidity than what carbonic acid would produce, and Option D indicates a very strong acid, which is not the case here.