Free Acids, Bases and Salts MCQs with Answers

201. Relative order of acidity of HF, HCl, HBr and HI acids is

• A. HCl > HBr > HI > HF
• B. HF > HCl > HBr > HI
• C. HI > HBr > HCl > HF
• D. HF > HI > HCl > HBr

Explanation: The correct order of acidity for the halogen acids is HI > HBr > HCl > HF. This trend is due to the decreasing bond strength between the hydrogen and halogen atoms down the group. As you move down the group in the periodic table, the size of the halogen atoms increases, which leads to weaker H-X bonds and thus greater acidity. HF is an exception because of its strong hydrogen bonding, which makes it less acidic compared to the other halogen acids despite having a stronger H-F bond.

202. Relative order of acidity of oxy acid is

• A. HClO > HClO2 > HClO3 > HClO4
• B. HClO4 > HClO3 > HClO2 > HClO
• C. HClO3 > HClO2 > HClO > HClO4
• D. HClO2 > HClO4 > HClO3 > HClO

Explanation: The acidity of oxyacids is influenced by the number of oxygen atoms bonded to the central atom. More oxygen atoms increase the electron-withdrawing effect, enhancing the acid's ability to donate protons. Therefore, the correct order of acidity for the given oxyacids is HClO4 > HClO3 > HClO2 > HClO. Option B correctly reflects this order, whereas the other options do not align with the principle that more oxygen atoms increase acidity.

203. pH of pure water at 25°C, kw = 1 x 10^-14

• A. 0
• B. 7
• C. 14
• D. None of the above

Explanation: The pH of pure water at 25°C is 7 because water is neutral at this temperature, meaning it has equal concentrations of hydrogen ions (H+) and hydroxide ions (OH-). The ion-product constant for water (Kw) at 25°C is 1 × 10-14, which is the product of the concentrations of H+ and OH-. Mathematically, this means [H+] = [OH-] = 1 × 10-7 M. The pH is calculated as the negative logarithm of the hydrogen ion concentration: pH = -log[H+] = 7. Options A and C are incorrect because they represent extreme acidic and basic conditions, respectively, and Option D is incorrect because one of the listed values (7) is correct.

204. The ionic product equilibrium constant is

• A. Ka
• B. Kb
• C. Kc
• D. Kw

Explanation: The ionic product equilibrium constant, represented by Kw, is defined for the self-ionization of water. It is the product of the concentrations of H+ and OH- ions in water, typically at 25°C, where Kw = [H+][OH-] = 1.0 × 10-14 mol2/L2. Kw is unique to the self-ionization of water, whereas Ka and Kb are used for acids and bases, and Kc is a general equilibrium constant.

205. The value of kw increases with temperature because the ionization of water

• A. Decreases
• B. Remains constant
• C. Increases
• D. None of the above

Explanation: The increase in the value of Kw with temperature is due to the endothermic nature of the ionization of water. As temperature rises, the equilibrium shifts to favor the formation of more ions (H+ and OH-), thereby increasing Kw. Option A is incorrect because ionization does not decrease with temperature. Option B is incorrect because if ionization remained constant, Kw would not change. Option D is incorrect as there is a valid explanation for the observed phenomenon.

206. The concentration of OH- ions in a certain household ammonia solution is 0.0025. This ammonia solution is

• A. Basic
• B. Acidic
• C. Neutral
• D. None of above

Explanation: The concentration of OH- ions in the ammonia solution is 0.0025 M, which is significantly higher than the concentration of H+ ions in a neutral solution (1.0 x 10-7 M). This indicates that the solution is basic. Option A is correct because a higher concentration of OH- ions signifies a basic solution. Option B is incorrect as an acidic solution would have a higher concentration of H+ ions. Option C is incorrect because a neutral solution would have equal concentrations of H+ and OH- ions, around 1.0 x 10-7 M. Option D is incorrect as the solution clearly fits into the category of being basic.

207. The pH of the tears is

• A. 7.0
• B. 7.4
• C. 7.8
• D. 8.2

Explanation: The correct pH of tears is approximately 7.4, which is slightly alkaline. This level helps to maintain eye health by providing an effective antibacterial environment while not causing irritation. A pH of 7.0 is neutral, which is lower than the typical pH of tears, potentially affecting their protective function. A pH of 7.8 or 8.2 would be too alkaline, which could lead to eye irritation or indicate an abnormal condition.

208. The pH of the 0.1 M HCl solution is

• A. 0.01
• B. 0.1
• C. 0.2
• D. 1

Explanation: The pH of a solution is determined by the negative logarithm of the hydrogen ion concentration (pH = -log[H+]). For a 0.1 M HCl solution, the concentration of hydrogen ions [H+] is 0.1 M because HCl is a strong acid and dissociates completely. Therefore, the pH = -log(0.1) = 1. Option A (0.01), Option B (0.1), and Option C (0.2) are incorrect as they do not reflect the correct calculation using the logarithmic pH formula.

209. The pH of the 1 M HCl is

• A. 0
• B. 0.1
• C. 1
• D. 0.2

Explanation: The pH of a solution is calculated as the negative logarithm (base 10) of the hydrogen ion concentration. In the case of 1 M HCl, which is a strong acid, it dissociates completely in water to produce 1 M of hydrogen ions. The pH is therefore -log(1) = 0. Other options are incorrect because they either suggest a misunderstanding of the pH scale or the behavior of strong acids in solution.

210. The sum of pH and pOH is equal to

• A. 1
• B. 14
• C. 2
• D. 13

Explanation: In aqueous solutions at 25°C, the sum of pH and pOH is always 14. This is derived from the ion product of water (Kw), where Kw = [H+][OH-] = 1 × 10-14. Therefore, pH + pOH = -log(Kw) = 14. Option B is correct as it reflects this standard value. The other options are incorrect because they do not match the established relationship between pH and pOH in water at 25°C.