Question

The equivalent weight of ${\text{S}}{{\text{O}}_2}$ in the given reaction ${\text{S}}{{\text{O}}_2}\, + \,{{\text{H}}_2}{\text{S}}\, \to \,{\text{S}}\,{\text{ + }}\,{{\text{H}}_2}{\text{O}}$ is:
A. $64$
B. $32$
C. $16$
D. $8$

Answer 1

Hint: We can determine the equivalent weight by diving the molecular weight with the valence factor. For sulphur dioxide, the valence factor is the number of electrons gained during the reaction. We will determine the change in the oxidation number of sulphur to determine the valence factor of sulphur dioxide.

Formula used: \[{\text{equivalent weight}}\,{\text{ = }}\,\dfrac{{{\text{Molecular weight}}}}{{{\text{valance}}\,{\text{factor}}}}\]

Complete answer:
Molecular weight is the product of equivalent weight and valence factor.
The formula to determine the equivalent weight is as follows:
\[{\text{equivalent weight}}\,{\text{ = }}\,\dfrac{{{\text{Molecular weight}}}}{{{\text{valance}}\,{\text{factor}}}}\]
The valence factor is the number of electrons gained or lost by a species or the oxidation number of the molecule.
Change in oxidation number of the sulphur is as follows:
The oxidation number of oxygen is $ - 2$ so, the oxidation number of sulphur in ${\text{S}}{{\text{O}}_2}$ is,
$x\, + \left( { - 2 \times 2} \right) = 0$
$\Rightarrow x\, = + 4$
So, the oxidation number of sulphur in ${\text{S}}{{\text{O}}_2}$ is, $ + 4$
The oxidation number of elemental sulphur is 0.
So, the change in oxidation number is,
$ = 0 - ( + 4)$
$ = 4$
So, the valence factor for sulphur dioxide is $4$.
Molar mass of sulphur dioxide is $64\,{\text{g/mol}}$ .
Substitute $64\,{\text{g/mol}}$ for molar mass of sulphur dioxide and $4$ for valence factor in equivalent formula.
\[\Rightarrow {\text{equivalent weight}}\,{\text{ = }}\,\dfrac{{{\text{64}}}}{4}\]
\[\Rightarrow {\text{equivalent weight}}\,{\text{ = }}\,16\]
So, the equivalent weight of ${\text{S}}{{\text{O}}_2}$ in the given reaction ${\text{S}}{{\text{O}}_2}\, + \,{{\text{H}}_2}{\text{S}}\, \to \,{\text{S}}\,{\text{ + }}\,{{\text{H}}_2}{\text{O}}$ is $16$.

Therefore, option (C) $16$ is correct.

Note:
In the case of acid, the valance factor is determined as the number of protons donated by the species. For some acids, for example potassium permanganate, the valence factor depends upon the medium. For the base, the number of protons accepted shows the valence factor. Sulphur dioxide is working as an oxidizing agent. So, it gets reduced means it gains electrons in the reaction.