Free Acids, Bases and Salts MCQs with Answers

101. pKa values of some acids are given below. Choose the weaker acid:

• A. HClO4(-10)
• B. HBr(-9)
• C. H2SO4(-3)
• D. HCl(-7)

Explanation: The strength of an acid is inversely proportional to its pKa value. A lower pKa value indicates a stronger acid. Among the given options, H2SO4 has the highest pKa value of -3, making it the weakest acid. In contrast, HClO4 with a pKa of -10 is the strongest, followed by HBr (-9), and then HCl (-7).

102. Acid reacts, with which of the following to produce salt?

• A. Metal oxide
• B. Hydroxides
• C. Metals
• D. All of the above

Explanation: The correct answer is All of the above. Acids can react with metal oxides and hydroxides through neutralization reactions, resulting in the formation of salt and water. Additionally, acids can react with certain metals, typically those above hydrogen in the reactivity series, to produce salt and hydrogen gas. This variety of reactions showcases the different pathways through which acids can produce salts. Hence, each of these interactions is a valid mechanism for salt formation, making Option D the correct choice. Although metal oxides, hydroxides, and metals can individually react with acids, it is their collective ability to do so that fits the context of the question best.

103. The following species cannot produce salt on reaction with acid.

• A. Carbonates
• B. Metals
• C. Non-metal oxides
• D. Metal Hydroxides

Explanation: The correct answer is Option B: Metals. Not all metals react with acids to form salts. For instance, noble metals like gold and platinum do not form salts when they react with acids due to their lack of reactivity. Option A, carbonates, reliably form salts when reacting with acids, alongside water and carbon dioxide. Option C, non-metal oxides, generally react with bases rather than acids to form salts. Option D, metal hydroxides, undergo neutralization reactions with acids to produce salts and water.

104. Which of the following is amphoteric substance?

• A. Na₂O
• B. H₂CO₃
• C. H₂O
• D. NH₄OH

Explanation: An amphoteric substance has the ability to act as both an acid and a base. Water (H₂O) is a classic example of an amphoteric substance because it can donate a proton, forming hydroxide ions (OH⁻), or accept a proton, forming hydronium ions (H₃O⁺). Therefore, it can behave as either an acid or a base depending on the reaction conditions. In contrast, Na₂O is a basic oxide, H₂CO₃ is primarily an acid, and NH₄OH is a weak base, none of which exhibit the dual functionality required to be considered amphoteric.

105. An acid-base reaction is the transfer of proton from an acid to a base. This is according to

• A. Arrhenius concept
• B. Bronsted-Lowry concept
• C. Lewis concept
• D. All of the above

Explanation: The correct answer is the Bronsted-Lowry concept, which specifically characterizes acids as proton donors and bases as proton acceptors, highlighting the transfer of a proton. The Arrhenius concept is focused on the formation of ions in aqueous solutions without explicit mention of proton transfer. The Lewis concept involves electron pair transfers, making it unrelated to proton transfer in acid-base reactions. Thus, only the Bronsted-Lowry concept correctly defines an acid-base reaction as a proton transfer.

106. In acid base reaction, how many conjugate pairs are formed?

• A. One
• B. Two
• C. Three
• D. Four

Explanation: In a standard acid-base reaction, two conjugate pairs are formed. The acid donates a proton to become a conjugate base, while the base accepts a proton to become a conjugate acid. Therefore, Option B is correct. Option A is incorrect as it underestimates the number of conjugate pairs. Options C and D are incorrect because they overestimate the number of conjugate pairs formed in a typical acid-base reaction.

107. Which of the following is weakest acid?

• A. HCN
• B. NH₄⁺
• C. H₂S
• D. H₂CO₃

Explanation: HCN is the correct answer because it has the lowest dissociation constant (Ka) among the given options, indicating it is the weakest acid. NH₄⁺ is the conjugate acid of NH₃ and, while it can act as an acid, it is not generally compared to molecular acids like HCN, H₂S, or H₂CO₃ in terms of acidity strength. H₂S and H₂CO₃ are both weak acids but have higher dissociation constants than HCN, meaning they donate protons more readily and are thus stronger acids than HCN.

108. 1.0 M solution of NaOH is mixed with 1.0 M solution of H₂SO₄+ the solution formed is:

• A. Acidic
• B. Basic
• C. Neutral
• D. Amphoteric

Explanation: In the reaction between NaOH and H₂SO₄, sulfuric acid is a diprotic acid, meaning it can donate two protons per molecule. To completely neutralize the acid, you would need two moles of NaOH for every mole of H₂SO₄. In this case, you have equal concentrations of NaOH and H₂SO₄, meaning there is not enough NaOH to fully neutralize the acid, resulting in an acidic solution. The other options are incorrect because the solution cannot be basic or neutral due to the insufficient amount of NaOH for complete neutralization, and the concept of amphoterism does not apply here.

109. Which of the following is not a base?

• A. NH₃ (Ammonia)
• B. BF₃ (Boron Trifluoride)
• C. KOH (Potassium Hydroxide)
• D. PH₃ (Phosphine)

Explanation: The correct answer is Option B: BF₃. Boron Trifluoride (BF₃) is not a base; it acts as a Lewis acid because it can accept a pair of electrons due to the electron deficiency of boron. Option A: NH₃ is a base because it can accept a proton to form NH₄⁺. Option C: KOH is a strong base as it dissociates in water to release OH⁻ ions. Option D: PH₃ can act as a base by donating a pair of electrons, although it is not as commonly recognized as a base as NH₃ or KOH.

110. Which of the following is strongest Lewis base?

• A. Ethene
• B. Benzene
• C. Ethyne
• D. All have same strength

Explanation: The strongest Lewis base is the one that can donate electrons most effectively. Among the given options, Ethyne is the strongest Lewis base because the carbon atoms in ethyne are sp-hybridized, making the electrons in the triple bond more available for donation. Ethene and Benzene do not have the same electron density available for donation. Ethene is less electron-rich due to sp2 hybridization, and benzene's aromaticity stabilizes its electron cloud, making it less likely to donate electrons. Hence, ethyne is the strongest Lewis base among the options.