### Correct Option: B. Phenolphthalein #### Detailed Explanation: In a titration involving sodium carbonate (Na₂CO₃) and hydrochloric acid (HCl), we are looking for an appropriate indicator that will signal the endpoint of the reaction. Sodium carbonate is a basic salt, and hydrochloric acid is a strong acid. The reaction can be represented as follows: \[ \text{Na}_2\text{CO}_3 (aq) + 2 \text{HCl} (aq) \rightarrow 2 \text{NaCl} (aq) + \text{H}_2\text{O} (l) + \text{CO}_2 (g) \] During this titration, we are aiming for complete neutralization, which occurs when all the carbonate ions (CO₃²⁻) have reacted with the hydrogen ions (H⁺) from the hydrochloric acid. **Why Phenolphthalein is the Correct Choice:** 1. **pH Range**: Phenolphthalein is a pH indicator that changes color in the pH range of approximately 8.2 to 10.0. In this titration, the solution starts as basic due to the presence of sodium carbonate. As HCl is added, the pH will decrease until it reaches around 7 (neutral) at the endpoint of the titration. However, the endpoint of the titration will be slightly above 7, where phenolphthalein will change from pink (basic) to colorless (neutral/acidic). 2. **Color Change**: The clear color change from pink to colorless provides a distinct visual cue that the endpoint has been reached, making it easy to identify when to stop adding the acid. 3. **Strong Acid vs. Weak Base**: Since sodium carbonate is a weak base and HCl is a strong acid, phenolphthalein is suitable because it will indicate the transition from a basic to a neutral solution effectively. #### Why the Other Options are Incorrect: **A. Litmus Paper**: - **Limitations**: Litmus paper is a general pH indicator that changes color at around pH 4.5 to 8.3. In this titration, the endpoint will be around pH 7, which is not within the effective range of litmus paper. Therefore, it would not provide a clear indication of the endpoint. **C. Methyl Orange**: - **pH Range**: Methyl orange changes color from red to yellow in the pH range of 3.1 to 4.4. Since the endpoint of the titration will be above pH 7, methyl orange would not be suitable as it would remain yellow (indicating a basic solution) even when the reaction is complete. **D. Universal Indicator**: - **Complexity**: While universal indicator can show a range of colors across a wide pH spectrum, it is less precise for determining the endpoint of a titration compared to phenolphthalein. The color change can be gradual and may not provide a clear, distinct endpoint, making it less effective for this specific titration. ### Summary of Key Points: - **Correct Indicator**: Phenolphthalein is the best choice for titrating sodium carbonate with hydrochloric acid due to its effective pH range and clear color change. - **Endpoint Identification**: The endpoint is indicated by a change from pink to colorless, signaling complete neutralization. - **Inappropriate Indicators**: Litmus paper, methyl orange, and universal indicator are less effective due to their pH ranges and color change characteristics. - **Understanding pH**: Knowing the pH range of indicators is crucial for selecting the right one for specific titrations.