In the reaction of sodium thiosulphate with `I_(2)` in aqueous medium, the equivalent mass of sodium sulphate

Options

A. molar mass of sodium thiosulphate
B. the average of molar masses of `Na_2 S_2 O_3 and I_2`
C. half the molar mass of sodium thisoulphate
D. molar mass of sodium thisoulphate * 2

Answer

To find the equivalent mass of sodium thiosulphate (Na2S2O3) in its reaction with iodine (I2), we can follow these steps:

### Step 1: Understand the Reaction
The reaction between sodium thiosulphate and iodine can be represented as:
\[ I_2 + Na_2S_2O_3 \rightarrow NaI + Na_2S_4O_6 \]

### Step 2: Balance the Reaction
To balance the reaction, we can write:
\[ I_2 + 2 Na_2S_2O_3 \rightarrow 2 NaI + Na_2S_4O_6 \]
This shows that 1 mole of I2 reacts with 2 moles of sodium thiosulphate.

### Step 3: Identify the Number of Electrons Transferred
In the reaction, iodine (I2) is reduced to iodide (I^-), and sodium thiosulphate (Na2S2O3) is oxidized. The half-reactions can be written as:
- For iodine:
\[ I_2 + 2e^- \rightarrow 2I^- \]
- For sodium thiosulphate:
\[ S_2O_3^{2-} \rightarrow S_4O_6^{2-} + 2e^- \]

From the half-reactions, we can see that 2 electrons are transferred for every 2 moles of sodium thiosulphate.

### Step 4: Calculate the Equivalent Weight
The equivalent weight (or equivalent mass) of a substance is calculated using the formula:
\[ \text{Equivalent Weight} = \frac{\text{Molar Mass}}{\text{n}} \]
where \( n \) is the number of electrons lost or gained per mole of the substance.

For sodium thiosulphate:
- Molar mass of Na2S2O3 = (2 × 23) + (2 × 32) + (3 × 16) = 46 + 64 + 48 = 158 g/mol.
- From the reaction, we see that 1 mole of sodium thiosulphate loses 1 electron.

Thus, the equivalent weight of sodium thiosulphate is:
\[ \text{Equivalent Weight} = \frac{158 \text{ g/mol}}{1} = 158 \text{ g/equiv} \]

### Step 5: Conclusion
The equivalent mass of sodium thiosulphate in the reaction with iodine is equal to its molar mass, which is 158 g/equiv.

### Final Answer
The equivalent mass of sodium thiosulphate is **158 g/equiv**.

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