Sodium thiosulphate is the laboratory reagent that is used in the iodometric and iodometric titration. This also finds used in photography.

Based on the above passage, answer the following question:

$I_{2} + 2 S_{2} O_{3}^{2 -} \to S_{4} O_{6}^{2 -} + 2 I^{-}$
Find the equivalent mass of $S_{2} O_{3}^{2 -}$ where, molar mass = $M$ .

M

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\frac{M}{2}

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\frac{M}{3}

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\frac{M}{4}

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Solution

The correct option is C $M$
Reaction:
$I_{2} + 2 S_{2} O_{3}^{2 -} \to S_{4} O_{6}^{2 -} + 2 I^{-}$
Equivalent mass of $S_{2} O_{3}^{2 -}$
Sulphur oxidation state change $= 2 (2.5 - 2) = 1$ .
Therefore equivalent weight of $S_{2} O_{3}^{2 -} = M / 1 = M$ .

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I_{2}

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I_{2}

i n s i t u

B

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2 S_{2} O_{3}^{2 -} + I_{2} \to S_{4} O_{6}^{2 -} + 2 I^{-}

S_{2} O_{3}^{2 -} + 2 B r_{2} + 5 H_{2} O \to 2 S O_{4}^{2 -} + 2 B r^{-} + 10 H^{+}

2 S_{2} O_{3}^{2 -} (R e d u c i n g a g e n t) \to S_{4} O_{6}^{2 -} + 2 e^{-}

I_{2} + 2 {S_{2} O_{3}}^{2 -} \to 2 I^{-} + {S_{4} O_{6}}^{2 -}

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