Question

The eq. wt. of iodine of mass M in, ${{I}_{2}}+2{{S}_{2}}O_{3}^{2-}\to 2{{I}^{-}}+{{S}_{2}}O_{6}^{2-}$ is:
(A) $M$
(B) $\dfrac{M}{2}$
(C) $\dfrac{M}{4}$
(D) none of these

Answer 1

Hint: Equivalent weight of a compound refers to the mass of the substance that reacts exactly with an arbitrarily fixed amount of another compound. Equivalent weight of a compound is given as:
\[\text{Equivalent weight = }\dfrac{\text{Molecular weight}}{\text{number of electrons lost or gained}}\]

Complete step by step answer:
To determine the equivalent weight of iodine in the reaction given below, we first need to find out the number of electrons gained by iodine.
     ${{I}_{2}}+2{{S}_{2}}O_{3}^{2-}\to 2{{I}^{-}}+{{S}_{2}}O_{6}^{2-}$
Change in oxidation state of I in the reactant and product side will give us the number of electrons gained by ${{I}_{2}}$.

Oxidation state is the charge obtained by the atom of an element when it loses or gains electrons while going from its free state to combined state with atoms of other elements.
Oxidation state of atoms in their elementary state is taken as zero. Therefore, the oxidation state of I in ${{I}_{2}}$ is 0.

Oxidation state of I in \[{{I}^{-}}\] is -1.
Change in oxidation state of I will then be, 0-(1) = 1.
Therefore, the number of gained by one I = 1
One molecule of \[{{I}_{2}}\] has two I atoms. Then, the total number of electrons gained by two I atoms = $2\times 1=2$
Molecular weight of ${{I}_{2}}$ is given as M.

Since, 2 electrons are gained per mole of ${{I}_{2}}$, thus, the equivalent weight of iodine will now be calculated by dividing its molecular weight by 2.
     \[\text{Equivalent weight of iodine = }\dfrac{M}{2}\]
So, the correct answer is “Option B”.

Note: Note that in the given reaction oxidation state of I is decreasing from 0 to -1, that means ${{I}_{2}}$ is undergoing reduction and thus, acting as an oxidizing agent. One equivalent of ${{I}_{2}}$ is oxidizing two equivalents of thiosulphate ${{S}_{2}}O_{3}^{2-}$ hence, mass of iodine is divided by 2 to obtain its equivalent weight.