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The best indicator for titrating 0.1 N ...

The best indicator for titrating 0.1 N `Na_2CO_3` against 0.1 N HCl is

A

methyl orange

B

litmus

C

phenolphthalein

D

potassium ferricyanide

Text Solution

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The correct Answer is:
To determine the best indicator for titrating 0.1 N Na2CO3 (sodium carbonate) against 0.1 N HCl (hydrochloric acid), we need to analyze the reactions that occur during the titration and the pH changes that take place. ### Step-by-Step Solution: 1. **Identify the Reactions**: - The titration of sodium carbonate with hydrochloric acid involves two main reactions: - **First Reaction**: \[ \text{Na}_2\text{CO}_3 + \text{HCl} \rightarrow \text{NaHCO}_3 + \text{NaCl} \] - **Second Reaction**: \[ \text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \] 2. **Determine the pH at the Equivalence Point**: - The first reaction produces sodium bicarbonate (NaHCO3), which is a weak acid. The second reaction produces carbonic acid (H2CO3), which can further dissociate in water. Therefore, at the equivalence point, the solution will be slightly acidic due to the presence of carbonic acid. 3. **Choose the Appropriate Indicator**: - The choice of indicator depends on the pH at the equivalence point. Since the equivalence point is in the acidic range, we need an indicator that changes color in this pH range. - **Phenolphthalein** is not suitable because it changes color in a basic medium (pH > 8.2) and would not indicate the endpoint accurately in this titration. - **Methyl Orange** is a suitable choice as it changes color from red (in acidic solutions) to yellow (in neutral to basic solutions) and has a transition range of pH 3.1 to 4.4, which is appropriate for this titration. 4. **Conclusion**: - The best indicator for titrating 0.1 N Na2CO3 against 0.1 N HCl is **Methyl Orange**.
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