An aqueous buffer solution contains HC2H3O2(aq) and NaC2H3O2(aq). Which of the following statements — Acids and Bases Chemistry Question

STEPS:

1. Identify the components of the buffer system:
The aqueous buffer solution is prepared by mixing a weak acid, acetic acid ($\text{HC}_2\text{H}_3\text{O}_2$), and its conjugate base, the acetate ion ($\text{C}_2\text{H}_3\text{O}_2^-$), which is supplied by the soluble salt sodium acetate ($\text{NaC}_2\text{H}_3\text{O}_2$).

2. Understand how the buffer components resist pH changes:
* The weak acid ($\text{HC}_2\text{H}_3\text{O}_2$) is available to react with and neutralize any small amount of strong base ($\text{OH}^-$) added to the system.
* The conjugate base ($\text{C}_2\text{H}_3\text{O}_2^-$) is available to react with and neutralize any small amount of strong acid ($\text{H}^+$) added to the system.

3. Analyze the chemical reaction that occurs when a small amount of strong acid ($\text{HCl}$) is added:
* $\text{HCl}$ is a strong acid that dissociates completely in water, releasing free hydronium/hydrogen ions ($\text{H}^+$) into the solution.
* Rather than remaining free in solution (which would sharply lower the pH), these added $\text{H}^+$ ions react with the conjugate base ($\text{C}_2\text{H}_3\text{O}_2^-$) to form the weak, neutral acetic acid molecules:
$$\text{H}^+(aq) + \text{C}_2\text{H}_3\text{O}_2^-(aq) \rightarrow \text{HC}_2\text{H}_3\text{O}_2(aq)$$

4. Determine the effect on pH:
Because the added strong acid ions are successfully converted into a weak acid species, the concentrations of free hydronium ions in solution change very little. As a result, the pH of the buffer solution is mitigated and decreases only slightly, identifying Option B as the correct choice.

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WHY_OTHERS_WRONG:

• Option A is incorrect: Adding a strong acid like $\text{HCl}$ introduces $\text{H}^+$ ions which would lower the pH of a solution, not increase it. Additionally, the spectator chloride anions ($\text{Cl}^-$) do not react with the acetate buffer components to mitigate pH.
• Option C is incorrect: When a strong base like $\text{NaOH}$ is added, the base contributes $\text{OH}^-$ ions. To neutralize these ions, they must react with the acid component of the buffer ($\text{HC}_2\text{H}_3\text{O}_2$) to form water and acetate, rather than reacting with the basic acetate anions ($\text{C}_2\text{H}_3\text{O}_2^-$).
• Option D is incorrect: While the option correctly identifies that the added $\text{OH}^-$ ions will react with the weak acid ($\text{HC}_2\text{H}_3\text{O}_2$) molecules, adding a base causes the pH of the solution to increase only slightly, not decrease.