Free Acids, Bases and Salts MCQs with Answers

51. Which of the following acids has the highest pKa value?

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

Explanation: The highest pKa value corresponds to the weakest acid. Among the given options, nitric acid, sulfuric acid, and perchloric acid are all very strong acids with highly negative pKa values. Chloric acid (HClO₃) is also a strong acid, but it is the weakest among this group, meaning it has the largest (least negative) pKa value

52. A solution with a pKa = -7 is a:

• A. Strong base
• B. Strong acid
• C. Neutral
• D. Weak acid

Explanation: The pKa is a measure of acid strength, with lower values indicating stronger acids. A very low or negative pKa value, such as -7, signifies a very strong acid that dissociates completely in solution.

53. Which of the following shows the limit of the Ka value for a strong acid?

• A. Kₐ < 10⁻³
• B. Kₐ=1 to 10⁻³
• C. Kₐ>1
• D. Kₐ= 0

Explanation: A large Ka value means the acid dissociates almost completely, shifting equilibrium far to the right. Hence, Ka > 1 indicates a strong acid, while weak acids have Ka << 1.

54. The pKb values of four solutions are given below. Which one is the most acidic?

• A. 3
• B. 4
• C. 5
• D. 2

Explanation: A higher pKb value indicates a weaker base. A weaker base corresponds to a stronger conjugate acid and thus a more acidic solution. Therefore, the solution with the highest pKb (5) will be the most acidic.

55. A very large pK ᵦ value indicates that the solution is:

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

Explanation: A very large pKb means the base is extremely weak.Its conjugate acid is therefore strong, making the resulting solution strongly acidic.

56. Which of the following relationships is correct for a conjugate acid-base pair?

• A. Ka=1/Kb
• B. Ka x Kb = Kw
• C. pKw=-logKw
• D. None of these

Explanation: For any conjugate acid-base pair in aqueous solution, the product of their dissociation constants equals the ionproduct constant of water:

57. A chemical reaction resulting in a change in the electric charge on the reacting particles may be called as:

• A. Add ion reaction
• B. Redox reaction
• C. Elimination reaction
• D. Chain reaction

Explanation: A 'redox reaction' involves the transfer of electrons between substances, leading to changes in oxidation states and electric charges of the reacting particles. This is correct as it directly involves changes in electric charge.

58. Acidic buffer can be prepared by mixing:

• A. Weak acid and weak base
• B. Weak acid and strong base
• C. Strong acid and strong base
• D. Strong acid and weak base

Explanation: An acidic buffer resists changes in pH and is prepared using a weak acid and its salt. Mixing a weak acid with a strong base produces the salt of the weak acid, which together with the remaining weak acid forms a buffer. Thus, an acidic buffer is made from a weak acid and strong base.

59. The conjugated acid of NH₃ is :

• A. NH₄⁺
• B. NH
• C. NH₂
• D. NH₂⁻

Explanation: A conjugate acid is formed when a base accepts a proton (H⁺). Ammonia (NH₃) acts as a base and accepts a proton to form NH₄⁺. Hence, the conjugate acid of NH₃ is ammonium ion.

60. Salts of weak bases react with strong acids to give a/an

• A. Basic solution
• B. Acidic solution
• C. Neutral solution
• D. None

Explanation: A salt derived from strong acid and weak base will dissolve in water to give a solution which is acidic due to anionic hydrolysis (or) A salt derived from strong acid and weak base will give acidic solution because in such solution [H3O+][H3O+] concentration will be more than [OH−]