Home / AP Chemistry: 4.8 Introduction to Acid – Base Reactions – Exam Style questions with Answer- MCQ

AP Chemistry: 4.8 Introduction to Acid – Base Reactions – Exam Style questions with Answer- MCQ

Questions 

 Samples of NaF(s) and\( NH_{4}\)Cl(s) are dissolved in separate beakers that each contain 100 mL of water. One of the salts produces a slightly acidic solution. Which of the following equations best represents the formation of the slightly acidic solution?

(A) $Na +(aq) + 2 H_{2}O(l) \rightleftharpoons NaOH(aq) + H_{3}O^{+}$(aq)
(B) $F ^{−}(aq) + H_{2}O(l) \rightleftharpoons HF(aq) + OH^{−}$(aq)
(C) $NH_{ 4}+(aq) + H_{2}O(l) \rightleftharpoons NH_3(aq) + H_{3}O^{+}$(aq)
(D)$Cl ^{−}(aq) + H_{2}O(l) \rightleftharpoons HCl(aq) + OH^{−}$(aq)

▶️Answer/Explanation

Ans: C

The correct answer is C. When \(NH_{4}^{+}\) ions are dissolved in water, they can act as a Bronsted-Lowry acid and donate a proton \(H^+\) to water molecules, forming \(NH_{3}(aq)\) and \(H_{3}O^{+}
(aq)\). This increases the concentration of \(H_{3}O^{+}\) ions in the solution, making it slightly acidic. Here is the reaction:

\(NH_{ 4}+(aq) + H_{2}O(I) \rightleftharpoons NH3(aq) + H_{3}0^{+}\)(aq)

This process is known as the hydrolysis of \(NH_{4}^{+}\) ions.

Questions

\(NH_{3}\) reacts with \(BF_{3}\) to form a single species. Which of the following structural diagrams is the most likely representation of the product of the reaction?

▶️Answer/Explanation

Ans: D

The reaction between ammonia (\(NH_{3}\)) and boron trifluoride (\(BF_{3}\)) forms a single species known as ammonia borane (\(NH_{3}BH_{3}\)).

The reaction can be represented as:

\[ NH_{3} + BF_{3} \rightarrow NH_{3}BH_{3} \]

In this reaction, the lone pair of electrons on the nitrogen atom (\(N\)) in \(NH_{3}\) forms a coordinate covalent bond with the boron atom (\(B\)) in \(BF_{3}\), resulting in the formation of a new compound, ammonia borane (\(NH_{3}BH_{3}\)).

This reaction is an example of a Lewis acid-base reaction, where the ammonia molecule acts as a Lewis base (electron pair donor) and the boron trifluoride molecule acts as a Lewis acid (electron pair acceptor).

Question

                               \(C_6H_5COOH(aq)+NaOH(aq)→C_6H_5COONa(aq)+H2O(l)\)

Which of the following identifies a conjugate acid-base pair in the reaction represented above?

    

▶️Answer/Explanation

Ans: B

 The conjugate base is the species that results from the loss of the proton \(H^+\) from the acid. In this reaction, \(C_6H_5COOH\) is a weak acid that donates an \(H^+\) to \(OH^−\) to form \(H_2O\) and the conjugate base, \(C_6H_5COO^−\).

Question

                               \(C_6H_5COOH(aq)+NaOH(aq)→C_6H_5COONa(aq)+H2O(l)\)

Which of the following identifies a conjugate acid-base pair in the reaction represented above?

    

▶️Answer/Explanation

Ans: B

 The conjugate base is the species that results from the loss of the proton \(H^+\) from the acid. In this reaction, \(C_6H_5COOH\) is a weak acid that donates an \(H^+\) to \(OH^−\) to form \(H_2O\) and the conjugate base, \(C_6H_5COO^−\).

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