A solution containing HCl and the weak acid HClO2 has a pH of 2.4. Enough KOH(aq) is added to the so — Acids and Bases Chemistry Question

STEPS:

1. Analyze the components in the initial solution: The starting solution contains hydrochloric acid ($\text{HCl}$) and chlorous acid ($\text{HClO}_2$). Hydrochloric acid is a strong acid that dissociates completely into $\text{H}^+(aq)$ and $\text{Cl}^-(aq)$ ions. Chlorous acid is a weak acid that only partially ionizes and exists in equilibrium with its conjugate base ($\text{ClO}_2^-$):
$$\text{HClO}_2(aq) \rightleftharpoons \text{H}^+(aq) + \text{ClO}_2^-(aq)$$
2. Understand the reaction of adding a strong base ($\text{KOH}$): Potassium hydroxide is a strong base that dissociates completely in water to release highly reactive hydroxide ($\text{OH}^-$) ions.
3. Analyze the chemical shift on the weak acid equilibrium: As $\text{OH}^-$ ions are added, they react with and consume the free $\text{H}^+(aq)$ ions in solution to form water:
$$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$$
According to Le Chatelier's Principle, the removal of $\text{H}^+(aq)$ causes the weak acid dissociation equilibrium to shift to the right to replenish the lost hydrogen ions:
$$\text{HClO}_2(aq) \rightarrow \text{H}^+(aq) + \text{ClO}_2^-(aq)$$
4. Represent the overall net neutralization reaction: We can combine these processes to show how the weak acid molecules react directly with the added base:
$$\text{HClO}_2(aq) + \text{OH}^-(aq) \rightarrow \text{ClO}_2^-(aq) + \text{H}_2\text{O}(l)$$
This reaction demonstrates that as the base is added, reactant molecules of the weak acid ($\text{HClO}_2$) are consumed, while product ions of the conjugate base ($\text{ClO}_2^-$) are generated.
5. Relate to the final pH state: The pH of the solution increases from an acidic $2.4$ to a highly basic $10.5$. In such a basic environment ($\text{pH} = 10.5$), the system lies far past the equivalence point of the weak acid titration. The chlorous acid will have lost its protons and will exist almost exclusively as dissolved $\text{ClO}_2^-(aq)$. Thus, the absolute amount of $\text{ClO}_2^-(aq)$ increases, making Option C the correct answer.

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WHY_OTHERS_WRONG:

• A is incorrect: Chloride ions ($\text{Cl}^-$) are spectator ions originating from the complete dissociation of the strong acid $\text{HCl}$. Because $\text{Cl}^-$ is an exceptionally weak conjugate base, it does not react with the added $\text{KOH}$ or participate in any acid-base equilibria, meaning its absolute amount remains completely unchanged.
• B is incorrect: The pH of the solution increases from $2.4$ to $10.5$. Because pH is calculated as $\text{pH} = -\log[\text{H}^+]$, an increase in pH represents a massive decrease in the concentration and total amount of free $\text{H}^+(aq)$ ions in the vessel.
• D is incorrect: The weak acid molecules ($\text{HClO}_2$) are consumed by reacting with the added hydroxide ions to form $\text{ClO}_2^-$ and water. Consequently, the amount of $\text{HClO}_2(aq)$ decreases rather than increases.