Question

In the reaction, ${I_2} + 2{S_2}{O_3}^{ - 2} \to 2{I^{ - 1}} + {S_4}{O_6}^{ - 2}$, equivalent mass of iodine is:
(A) Equal to its molecular mass
(B) 1/2 the molecular mass
(C) 1/4 the molecular mass
(D) Twice the molecular mass

Answer 1

Hint: Equivalent weight is the molecular weight divided by the n-factor of the reaction. So, find the n-factor by first finding out the change in oxidation state of iodine.

Complete step by step answer:
-First we need to see what equivalent weight is.
Equivalent weight is defined as the weight of a given substance that will combine with a given quantity of a given substance. It can also be defined as the mass obtained by dividing the molecular weight of a substance by its acidity or basicity (n-factor).
 $Eq.wt. = \dfrac{{Mol.wt.}}{{n - factor}}$
-The given reaction is :
${I_2} + 2{S_2}{O_3}^{ - 2} \to 2{I^{ - 1}} + {S_4}{O_6}^{ - 2}$
-In this reaction ${I_2}$ acts as an oxidising agent because it oxidises the thiosulfate anion (${S_2}{O_3}^{ - 2}$) into tetrathionate anion (${S_4}{O_6}^{ - 2}$), while thiosulfate anion (${S_2}{O_3}^{ - 2}$) acts as a reducing agent because it reduces ${I_2}$ to ${I^ - }$ anion.
Here iodine is reduced from ${I_2}$ to ${I^ - }$: ${I_2} \to 2{I^ - }$ , and the oxidation state changes from 0 in ${I_2}$ to (-1) in ${I^ - }$ .
So, the electron change or change in oxidation state for 1 atom of iodine is = 1
-Let us now calculate the n-factor for iodine in this reaction.
n-factor = number of I atoms × electron change ( or change in oxidation number)
= 2 × 1
= 2
So, the n-factor for iodine in this reaction would be 2.
-Now we will calculate the equivalent weight. Let the molecular mass of iodine be M.
$Eq.wt. = \dfrac{M}{2}$

So, the correct option is: (B) 1/2 the molecular mass

Note: When we calculate the n-factor for a molecule containing 2 or more atoms the change in electron number should be multiplied with the stoichiometric amount of atoms to obtain the n-factor of the molecule. Also identify the oxidising and reducing agents correctly.
Also thiosulphate is an oxoanion of sulfur with tetrahedral geometry and along with sodium atom it is used as a prime treatment for cyanide poisoning along with hydroxocobalamin and it also activates thiosulfate sulfurtransferase (TST) in mitochondria, which protects us from obesity and type II diabetes.