### Correct Option: B. Phenolphthalein #### Detailed Explanation: In the titration of sodium carbonate (Na₂CO₃) against 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: \[ \text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \uparrow \] 1. **Understanding the pH Change**: - Sodium carbonate in solution has a basic pH (around 10-11). - As we add hydrochloric acid, the pH will gradually decrease until it reaches neutral (pH 7) and then slightly acidic (pH < 7) as we continue to add HCl. 2. **Choosing the Right Indicator**: - An ideal indicator for this titration should change color around the pH range where the solution transitions from basic to acidic. - **Phenolphthalein** is colorless in acidic solutions and turns pink in basic solutions, changing color around pH 8.2 to 10.0. This makes it suitable for detecting the endpoint of the titration when the solution transitions from basic to neutral. 3. **Why Other Options Are Incorrect**: - **A. Litmus Paper**: - Litmus paper is a simple indicator that changes color at around pH 4.5 (red in acid, blue in base). It is not precise enough for this titration because it does not provide a clear endpoint for the transition from basic to neutral. - **C. Methyl Orange**: - Methyl orange changes color from red to yellow between pH 3.1 and 4.4. This is too low for the titration of sodium carbonate, as the solution will not reach this acidic level until after the endpoint has been surpassed. Therefore, it would not accurately indicate the endpoint of the titration. - **D. Universal Indicator**: - While universal indicator provides a range of colors for different pH levels, it is not as precise as phenolphthalein for this specific titration. The color change can be gradual and may not provide a clear endpoint, making it less effective for determining the exact moment of neutralization. #### Summary of Key Points: - **Phenolphthalein** is the best choice for indicating the endpoint in the titration of sodium carbonate against hydrochloric acid due to its color change in the relevant pH range. - **Litmus paper** is too general and lacks precision for this titration. - **Methyl orange** is unsuitable as it changes color at too low a pH. - **Universal indicator** is less effective for pinpointing the endpoint compared to phenolphthalein. ### Revision Summary: - **Indicator Choice**: Use phenolphthalein for titrations involving strong acids and weak bases. - **pH Range**: Phenolphthalein changes color around pH 8.2 to 10.0, suitable for sodium carbonate. - **Avoid Methyl Orange**: It is too acidic for this titration. - **Litmus Paper Limitations**: Not precise enough for determining the endpoint in titrations.