Question

In the reaction, $Fe{S_2} + KMn{O_4} + {H^ + } \to F{e^{3 + }} + S{O_2} + M{n^{2 + }} + {H_2}O$ , the equivalent mass of $Fe{S_2}$ would be equal to $\_\_\_\_\_\_\_$
A: Molar mass
B:\[\dfrac{{{\text{Molar Mass}}}}{{10}}\]
C: \[\dfrac{{{\text{Molar Mass}}}}{{11}}\]
D: \[\dfrac{{{\text{Molar Mass}}}}{{13}}\]

Answer 1

Hint: Equivalent mass is the mass of $1$ equivalent which is equal to \[\dfrac{{{\text{Molar Mass}}}}{{{\text{Valency}}}}\] .Valency is the charge on an atom if the molecule was ionic. It can be replaced by oxidation number or change of oxidation number.
Formula used: ${\text{Equivalent Mass}} = \dfrac{{{\text{Molar Mass}}}}{{{\text{Valency}}}}$.

Complete step by step answer:
$Fe{S_2} + KMn{O_4} + {H^ + }\xrightarrow{{}}F{e^{3 + }} + S{O_2} + M{n^{2 + }} + {H_2}O$
Oxidation state of $Fe$ in $Fe{S_2}$ is $ + 2$
Oxidation state of $S$ in $Fe{S_2}$ is $ - 1$
Oxidation state of $Fe$ after reaction takes place is $ + 3$
Oxidation state of $S$ in $S{O_2}$ is $ + 4$
$F{e^{2 + }}\xrightarrow{{}}F{e^{3 + }} + {e^ - }$
Number of electron lost is $1$
${S^{ - 1}}\xrightarrow{{}}{S^{ + 4}} + 5{e^ - }$
Number of electron lost is $5$ but in reaction $2$ sulphur atoms are present therefore total electron loss will be $10$
1 molecule of $Fe{S_2}$ lost total $10 + 1 = 11$ electrons therefore change in oxidation number is $11$ as we know that
${\text{Equivalent Mass}} = \dfrac{{{\text{Molar Mass}}}}{{{\text{Valency}}}}$
                              \[ = \dfrac{{{\text{Molar Mass}}}}{{11}}\]
Therefore the answer to this question is option C that is \[\dfrac{{{\text{Molar Mass}}}}{{11}}\].

Additional information: Redox reaction is a type of chemical reaction in which the oxidation states of atoms are changed. Redox reactions are characterized by transfer of electrons between chemical species in which $1$ species undergoes oxidation and others undergo reduction. Chemical species from which electrons are removed are said to have been oxidized; on the other hand species which gain electrons are said to have been reduced. In other words we can say that oxidation is loss of electron and reduction is gain of electron. During the combustion of wood with molecular oxygen, the oxidation state of carbon atoms in wood increases and that of oxygen atoms decreases as carbon dioxide $C{O_2}$ and water ${H_2}O$ are formed this means oxygen undergoes reduction and carbon undergoes oxidation. Also we can say that oxygen is an oxidizing agent and carbon is a reducing agent in this case. Oxidizing agents are the species which itself undergoes a reduction but oxidizes others similarly reducing agents are the species which itself undergoes oxidation and reduces others.


Note:
Equivalent mass is also called gram equivalent.
Don’t get confused with oxidizing and reducing agents. Oxidizing agent itself undergoes reduction and reducing agent itself undergoes oxidation.