Question

In the reaction of sodium thiosulphate with ${I_2}$ in aqueous medium the equivalent weight of sodium thiosulphate is equal to:
(A) Molar mass of sodium thiosulphate
(B) The average molar mass of $N{a_2}{S_2}{O_3}$ and ${I_2}$
(C) Half the molar mass of sodium thiosulphate
(D) Twice of molar mass of sodium thiosulphate

Answer 1

Hint- In order to solve the given problem and find the equivalent mass of sodium thiosulphate in the reaction. First we will understand the basic definition of equivalent and molar mass and the concept how to find each of them. Further on the basis of the chemical reaction of sodium thiosulphate with iodine.

Complete step by step solution:
> First let us understand the basic concept behind molar mass and equivalent mass of any particular compound.
> Molar mass: Molar mass is the mass of any particular given compound is the mass of one mole of that molecule. One mole of the compound consists of Avogadro’s number of similar molecules in it.
> Equivalent weight: equivalent weight is given on the basis of molecular weight of the compound as well as number of electrons that a molecule gains or losses in a particular chemical reaction. The equivalent weight of a molecule is calculated by dividing the molecular weight of the same compound with the number of electrons lost or gained.
> Now as we know the concept behind molar weight and equivalent weight, so let us now proceed further to solve the problem.
> We will need the reaction between the iodine and sodium thiosulphate to find the number of electrons lost or gained in a molecule to find the equivalent weight.
> The chemical reaction between iodine and sodium thiosulphate is given as:
\[{I_2} + 2N{a_2}{S_2}{O_3} \to 2NaI + N{a_2}{S_4}{O_6}\]
> In order to find the number of electrons lost or gained in the reaction let us see the half reaction of iodine and thiosulphate ions.
The half reaction is as follows:
\[{I_2} + 2{e^ - } \to 2{I^ - } \\
  2{S_2}O_3^{2 - } \to {S_4}O_6^{2 - } + 2{e^ - } \\ \]
From the given reaction we can see that for 2 molecules of sodium thiosulphate, 2 electrons are lost by sodium thiosulphate.
So, one molecule of sodium thiosulphate will lose one electron eventually.
Therefore, the number of electrons lost is 1. So, the equivalent weight of sodium thiosulphate in terms of molar weight is give as:
\[\Rightarrow {\text{Equivalent weight}} = \dfrac{{{\text{Molecular weight}}}}{{{\text{Number of electrons lost}}}} \\
   \Rightarrow {\text{Equivalent weight}} = \dfrac{{{\text{Molecular weight}}}}{{\text{1}}} \\\Rightarrow {\text{Equivalent weight}} = {\text{Molecular weight}} \\ \]
Hence, the equivalent weight of sodium thiosulphate is equal to molar mass of sodium thiosulphate.
So, the correct answer is option A.

Note- In order to solve such types of problems students must be aware of general reactions between the compounds. In the problem we needed to find the relationship between different weights, so we did not find their numerical value. Students must remember the method to find the molar weight of a molecule by the help of molar weight of elements.