Question

The equivalent weight of $\text{Na}_2\text{S}_2\text{O}_3$ (Gram molecular weight = $M$) in the given reaction is:
$$\text{I}_2 + 2 \text{Na}_2\text{S}_2\text{O}_3 \rightarrow 2 \text{NaI} + \text{Na}_2\text{S}_4\text{O}_6$$

• A. M/2
• B. M
• C. 2M
• D. M/4

Answer 1

The Correct Option is B

The reaction involves: $\text{I}_2 + 2\text{Na}_2\text{S}_2\text{O}_3 \rightarrow 2\text{NaI} + \text{Na}_2\text{S}_4\text{O}_6$.

Here, each molecule of $\text{Na}_2\text{S}_2\text{O}_3$ donates 1 electron, leading to a total of 2 electrons being transferred per molecule of $\text{I}_2$.

Oxidation states of sulfur: In $\text{Na}_2\text{S}_2\text{O}_3$, the average oxidation state of sulfur is +2. During the reaction, sulfur in $\text{Na}_2\text{S}_2\text{O}_3$ is partially oxidized to $\text{Na}_2\text{S}_4\text{O}_6$, where the average oxidation state becomes +2.5.

Definition of equivalent weight: The equivalent weight of a substance is given by: $\text{Equivalent Weight} = \frac{\text{Molecular Weight}}{n}.$ Here, n represents the number of electrons transferred per molecule in the reaction.

Calculation: For $\text{Na}_2\text{S}_2\text{O}_3$, $n = 1$, because each molecule transfers 1 electron. Therefore, the equivalent weight of $\text{Na}_2\text{S}_2\text{O}_3$ is equal to its molecular weight, M. Thus, the equivalent weight of $\text{Na}_2\text{S}_2\text{O}_3$ in this reaction is M.

Answer 2

Correct Answer:

Option 2: M

Explanation:

1. Analyze the Reaction:

I₂ + 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆

2. Determine the Change in Oxidation State:

In Na₂S₂O₃, the average oxidation state of sulfur is +2.

In Na₂S₄O₆, the average oxidation state of sulfur is +2.5.

3. Calculate the Change per Molecule:

Each S atom in Na₂S₂O₃ changes its oxidation state from +2 to +2.5, a change of 0.5.

There are two sulfur atoms in Na₂S₂O₃, so the total change in oxidation state per molecule of Na₂S₂O₃ is 2 * 0.5 = 1.

4. Determine the n-factor:

The n-factor (number of electrons transferred per molecule) is equal to the total change in oxidation state, which is 1.

5. Calculate the Equivalent Weight:

Equivalent weight = Molecular weight / n-factor

Equivalent weight = M / 1 = M