A 20. mL sample of 0.50 M HC2H3O2(aq) is titrated with 0.50 M NaOH(aq). Which of the following best — Acids and Bases Chemistry Question

STEPS:

1. Identify the chemical nature of the reactants:
* $\text{HC}_2\text{H}_3\text{O}_2$ (acetic acid) is a weak acid. Unlike strong acids, weak acids only partially ionize in aqueous solution and exist primarily as intact, neutral molecules in water. Therefore, in net ionic equations, they must be represented in their intact molecular form rather than as dissociated ions.
* $\text{NaOH}$ (sodium hydroxide) is a strong base. Strong bases are strong electrolytes that dissociate completely into free ions in aqueous solution:
$$\text{NaOH}(aq) \rightarrow \text{Na}^+(aq) + \text{OH}^-(aq)$$

2. Formulate the complete molecular equation:
When acetic acid is titrated with sodium hydroxide, a neutralization reaction occurs to produce liquid water and the soluble salt sodium acetate:
$$\text{HC}_2\text{H}_3\text{O}_2(aq) + \text{NaOH}(aq) \rightarrow \text{H}_2\text{O}(l) + \text{NaC}_2\text{H}_3\text{O}_2(aq)$$

3. Write the complete ionic equation:
Express all strong electrolytes (the strong base $\text{NaOH}$ and the highly soluble salt $\text{NaC}_2\text{H}_3\text{O}_2$) as dissociated ions, while keeping the weak acid and liquid water as molecules:
$$\text{HC}_2\text{H}_3\text{O}_2(aq) + \text{Na}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l) + \text{Na}^+(aq) + \text{C}_2\text{H}_3\text{O}_2^-(aq)$$

4. Identify and cancel spectator ions:
* Spectator ions are ions that appear in the exact same state on both the reactant and product sides of the chemical equation.
* In this reaction, the sodium ion ($\text{Na}^+$) does not participate in the chemical change and remains a spectator.
* Subtracting $\text{Na}^+$ from both sides leaves the net ionic equation:
$$\mathbf{\text{HC}_2\text{H}_3\text{O}_2(aq) + \text{OH}^-(aq) \rightarrow \text{C}_2\text{H}_3\text{O}_2^-(aq) + \text{H}_2\text{O}(l)}$$

5. Conclude:
This equation correctly represents the actual reacting species (intact weak acid molecules and free hydroxide ions) and the species produced (acetate ions and water). This matches Option C.

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WHY_OTHERS_WRONG:

• Option A is incorrect: This is the net ionic equation for the reaction between a strong acid and a strong base. Because acetic acid is a weak acid, it does not fully dissociate to yield free hydronium/hydrogen ($\text{H}^+$) ions in solution.
• Option B is incorrect: This equation incorrectly represents the weak acid as being completely dissociated into $\text{H}^+$ and $\text{C}_2\text{H}_3\text{O}_2^-$ on the reactant side. It also incorrectly represents the soluble salt product, sodium acetate, as an undissociated molecular species ($\text{NaC}_2\text{H}_3\text{O}_2$) on the product side.
• Option D is incorrect: This is a mixed molecular/ionic equation that fails to cancel the sodium spectator ions ($\text{Na}^+$) from both sides of the equation. A proper net ionic equation must show only the species undergoing a chemical change.