A : `I_(2)` is a mild oxidising agent.
R : `I_(2)` can be used for titrating sodium thiosulphate.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4).

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion and Reason**:
- The assertion states that \( I_2 \) (iodine) is a mild oxidizing agent.
- The reason provided is that \( I_2 \) can be used for titrating sodium thiosulfate (\( Na_2S_2O_3 \)).

2. **Definition of an Oxidizing Agent**:
- An oxidizing agent is a substance that oxidizes another substance by accepting electrons and, in the process, gets reduced itself.

3. **Reaction of Iodine with Sodium Thiosulfate**:
- The reaction between iodine and sodium thiosulfate can be represented as:
\[ I_2 + 2 Na_2S_2O_3 \rightarrow Na_2S_4O_6 + 2 NaI \]

4. **Determining Oxidation States**:
- In \( I_2 \), iodine is in its elemental state, so its oxidation state is 0.
- In sodium thiosulfate (\( Na_2S_2O_3 \)), the oxidation state of iodine after the reaction is \(-1\) in \( NaI \).
- For sulfur in \( Na_2S_2O_3 \):
- Let the oxidation state of sulfur be \( x \).
- The equation becomes:
\[ 2(+1) + 2x + 3(-2) = 0 \implies 2 + 2x - 6 = 0 \implies 2x = 4 \implies x = +2 \]
- For sulfur in \( Na_2S_4O_6 \):
- Let the oxidation state of sulfur be \( y \).
- The equation becomes:
\[ 2(+1) + 4y + 6(-2) = 0 \implies 2 + 4y - 12 = 0 \implies 4y = 10 \implies y = +2.5 \]

5. **Analyzing the Changes in Oxidation States**:
- Iodine changes from 0 (in \( I_2 \)) to \(-1\) (in \( NaI \)), indicating that iodine is reduced.
- Sulfur changes from +2 (in \( Na_2S_2O_3 \)) to +2.5 (in \( Na_2S_4O_6 \)), indicating that sulfur is oxidized.

6. **Conclusion**:
- Since \( I_2 \) causes the oxidation of sulfur and is itself reduced, it confirms that \( I_2 \) is indeed an oxidizing agent.
- Furthermore, since it does not strongly oxidize other substances, it is classified as a mild oxidizing agent.

7. **Final Evaluation of Assertion and Reason**:
- Both the assertion (that \( I_2 \) is a mild oxidizing agent) and the reason (that \( I_2 \) can be used for titrating sodium thiosulfate) are true.
- The reason correctly explains the assertion.

### Final Answer:
Both assertion and reason are true, and the reason is the correct explanation of the assertion.

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