Free Acids, Bases and Salts MCQs with Answers

221. Which of the following is not base according to Lux-Flood concept?

• A. CaO
• B. MgO
• C. PbO
• D. SiO2

Explanation: This question tests the Lux-Flood concept in inorganic chemistry, critical for medical students understanding chemical behavior in physiological contexts. SiO2 is completely correct as it is not a base, unlike the basic oxides CaO, MgO, and PbO. Selecting other options risks misunderstanding acid-base chemistry.

222. Which of the following is not acid according to Lux-Flood concept?

• A. CaO
• B. CO2
• C. SO3
• D. SiO2

Explanation: The Lux-Flood concept defines acids as species that can accept oxide ions, while bases are those that can donate oxide ions. CaO, a basic oxide, can donate oxide ions, thus acting as a base rather than an acid in this context. In contrast, CO2, SO3, and SiO2 can accept oxide ions, making them acidic according to the Lux-Flood concept. Therefore, the correct answer is CaO, which is not an acid in this framework.

223. Which of the following is not soft acid according SHAB concept?

• A. K+
• B. Cu+
• C. Ag+
• D. Au+

Explanation: The HSAB (Hard and Soft Acids and Bases) concept classifies acids and bases as hard or soft. Hard acids have high charge density and less polarizability, while soft acids have low charge density and are more polarizable. K+ is a hard acid due to its small size and high charge density, making it different from the other options, which are soft acids. Cu+, Ag+, and Au+ are soft acids because they have lower charge density and greater polarizability, allowing them to form stable complexes with soft bases.

224. Which of the following is not hard acid according SHAB concept?

• A. Cu+
• B. Li+
• C. K+
• D. Na+

Explanation: According to the Hard and Soft Acids and Bases (HSAB) concept, hard acids are characterized by small size, high charge density, and low polarizability. Cu+ is a soft acid because it is more polarizable and does not have the high charge density typical of hard acids. In contrast, Li+, K+, and Na+ are all hard acids due to their small sizes and high charge densities, which make them less polarizable.

225. Which of the following is not soft base according SHAB concept?

• A. F-
• B. CN-
• C. H-
• D. R-

Explanation: The correct answer is F- because it is a hard base according to the SHAB concept. Hard bases are characterized by small size and high electronegativity, which is true for the fluoride ion. In contrast, CN-, H-, and R- are larger with lower charge densities, categorizing them as soft bases.

226. Which of the following is not hard base according SHAB concept?

• A. NH3
• B. N2H4
• C. RNH2
• D. I-

Explanation: According to the SHAB (Soft and Hard Acids and Bases) concept, hard bases are typically small, have high charge density, and low polarizability. NH3, N2H4, and RNH2 all fit these criteria, making them hard bases. On the other hand, I- is large and highly polarizable, which classifies it as a soft base. Thus, I- is not a hard base.

227. The Ka value of HCl is 107 what is its pKa value?

• A. 107
• B. -7
• C. 7
• D. Log 7

Explanation: To find the pKa value from the Ka value, use the formula pKa = -log(Ka). Given Ka = 107, the calculation becomes pKa = -log(107) = -7. This is why Option B is correct. Option A incorrectly implies no negative log transformation, Option C suggests a smaller Ka value, and Option D misunderstands the use of logarithms in this context.

228. Which of the following acid have highest pKa value?

• A. HClO3
• B. HNO3
• C. H2SO4
• D. HClO4

Explanation: The pKa value of an acid is inversely related to its strength; the higher the pKa, the weaker the acid. Among the given options, HClO3 is a strong acid but not as strong as HNO3 or H2SO4, which have lower pKa values. HClO4 is an even stronger acid than HClO3. Therefore, HClO3 has the highest pKa value in this list.

229. Which of the following expression is correct?

• A. Ka × Kb = pKw
• B. Ka × Kb = 14
• C. Ka × Kb = 10-14
• D. Ka × Kb = 1014

Explanation: The correct relationship between the ionization constants of an acid and its conjugate base is given by Ka × Kb = Kw. At 25°C, the ionization constant of water (Kw) is 10-14. Therefore, the expression Ka × Kb = 10-14 is correct. Option A is incorrect because it confuses Kw with its logarithmic form pKw. Option B is incorrect as it incorrectly equates the product to 14 instead of 10-14. Option D is incorrect because it incorrectly displays the product as 1014, which is the inverse of the correct value.

230. pKa value of carbonic acid is 6.4 what is pKb value of its conjugate base?

• A. 14
• B. 7.6
• C. 6.4
• D. 10

Explanation: The key to solving this question is understanding the relationship between the pKa of an acid and the pKb of its conjugate base, which is given by the equation: pKa + pKb = 14. For carbonic acid with a pKa of 6.4, the pKb of its conjugate base can be calculated as 14 - 6.4 = 7.6. Therefore, the correct answer is Option B: 7.6. Other options do not correctly apply this relationship.