Question

The equivalent weight of iron in $F{{e}_{2}}{{O}_{3}}$ would be:
(A) 18.6
(B) 26.66
(C) 56
(D) 112

Answer 1

In order to obtain the equivalent weight of iron obtained by reduction of the ferric ion in ferric oxide. The oxidation state or the moles of electrons required for reduction are taken into consideration.

Complete step by step answer:
Equivalent weight is defined as the weight of substance that contains or reacts with
1.0078g of hydrogen or 8g of oxygen, or 35.45g of chlorine.
Equivalent weight is the mass of one equivalent, that is, the mass of the substance which combines or displaces a fixed quantity of another substance.

In ferric oxide, $F{{e}_{2}}{{O}_{3}}$, as the ferric ions $F{{e}^{+3}}$ gain electrons in order to reduce to iron. Thus, in order to find the equivalent weight of Fe, the atomic mass of Fe is divided by the valency of Fe in the compound.
$2F{{e}_{2}}{{O}_{3}}\to 4Fe+3{{O}_{2}}$

On calculating the oxidation state of Fe, taking it equal to X. Then,
$2X+(3\times (-2))=0$
$2X-6=0$
$X=(+3)$

We see, $F{{e}_{2}}{{O}_{3}}$ has $2F{{e}^{+3}}$ and $3{{O}^{-2}}$ ions, thus, we require three moles of electrons to reduce $F{{e}^{+3}}$ to $Fe$.

The atomic weight of iron = 55.8 g/mol
Then, the equivalent weight of iron in $F{{e}_{2}}{{O}_{3}}$ is given by
$\text{equivalent}\,\text{weight}\,\text{of }\text{Fe}\,\text{=}\dfrac{\text{atomic}\,\text{weight}\,\text{of}\,\text{Fe}}{\text{valency}\,\text{of}\,\text{iron}}$
$=\dfrac{55.8}{3}=\text{ }18.6$
So, the correct answer is “Option A”.

Additional Information:
Equivalent weight basically depends on the reaction involved.
- For instance, in Neutralization Reaction, it is the weight of a compound that contains one equivalent of a proton (for acid) or hydroxide (for base).
Then, the equivalent weight of an acid is the weight of acid which contains one gram of replaceable hydrogen atoms.
$\text{Equivalent weight }\,\text{= }\dfrac{\text{Molecular weight of acid}}{\text{Basicity }\left( \text{which is number of hydrogens replaceable by base} \right)}\text{ }$
Whereas, the equivalent weight of a base is the weight of the substance, which contains one replaceable hydroxyl group.
$\text{Equivalent weight }\,\text{= }\dfrac{\text{Molecular weight of base}}{\text{Acidity }\left( \text{which is number of hydroxides replaceable by acid} \right)}\text{ }$
- In Precipitation Reactions, Equivalent weight of a salt is the gram molecular weight of the salt divided by the valency of the reacting ions.

Note: It has dimensions and unit of mass, thus, also called as the gram equivalent. Here, in this reaction, it simply involved the reduction process with the gain of moles of electrons.