C9H8O4(aq) + NaOH(aq) → C9H7O4-(aq) + Na+(aq) + H2O(l) The reaction represented above occurs when 2. — Acids and Bases Chemistry Question

STEPS:

1. Identify the type of reaction and species involved:
The reaction represents the titration of acetylsalicylic acid ($\text{C}_9\text{H}_8\text{O}_4$), which is a weak monoprotic acid, with sodium hydroxide ($\text{NaOH}$), a strong base. The chemical equation is:
$$\text{C}_9\text{H}_8\text{O}_4(aq) + \text{OH}^-(aq) \rightarrow \text{C}_9\text{H}_7\text{O}_4^-(aq) + \text{H}_2\text{O}(l)$$

2. Understand the behavior of a weak acid-strong base titration:
When a strong base is added to a weak acid, the neutralization reaction goes essentially to completion because the hydroxide ions ($\text{OH}^-$) have a very high affinity for protons and will strip them from the weak acid molecules.

3. Apply the definition of the equivalence point:
The equivalence point of a titration is reached when the number of moles of titrant added ($\text{OH}^-$) is stoichiometrically equal to the initial number of moles of analyte ($\text{C}_9\text{H}_8\text{O}_4$) in the flask. Here, exactly $2.00 \times 10^{-4}\text{ mol}$ of $\text{NaOH}$ has been added to neutralize the $2.00 \times 10^{-4}\text{ mol}$ of $\text{C}_9\text{H}_8\text{O}_4$.

4. Determine the dominant species at the equivalence point:
Because the neutralization reaction goes virtually to completion, almost all of the starting weak acid ($\text{C}_9\text{H}_8\text{O}_4$) has reacted with $\text{OH}^-$ and has been converted into its conjugate base, the acetylsalicylate ion ($\text{C}_9\text{H}_7\text{O}_4^-$).

5. Compare the final concentrations:
At the equivalence point, the flask contains a solution of the conjugate base salt. Only an infinitesimally small fraction of the conjugate base will hydrolyze water to reform the weak acid. Thus, the concentration of the conjugate base ($[\text{C}_9\text{H}_7\text{O}_4^-]$) is vastly greater than the concentration of the remaining unionized weak acid ($[\text{C}_9\text{H}_8\text{O}_4]$).

This mathematically means that $[\text{C}_9\text{H}_8\text{O}_4]$ is less than $[\text{C}_9\text{H}_7\text{O}_4^-]$, which corresponds to Option B.

*

WHY_OTHERS_WRONG:

• Option A is incorrect: This statement claims that there is more weak acid than conjugate base at the equivalence point. This relationship only occurs at the very beginning of the titration (before reaching the half-equivalence point), where very little base has been added.
• Option C is incorrect: $[\text{C}_9\text{H}_8\text{O}_4]$ is equal to $[\text{C}_9\text{H}_7\text{O}_4^-]$ at the half-equivalence point of the titration, where exactly half of the weak acid has been neutralized. At this point, $\text{pH} = \text{p}K_a$ of the weak acid. At the true equivalence point, the acid has been completely converted to its conjugate base.
• Option D is incorrect: The relationship can be easily determined. The standard behavior of weak acid-strong base neutralizations guarantees that conversion to the conjugate base is nearly complete at the equivalence point, regardless of the starting concentrations or the specific $K_a$ value of the weak acid.