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Q.
Oxidation number of $Fe$ in $Fe _{3} O _{4}$ are:
Haryana PMTHaryana PMT 2001
Solution:
$Fe _{3} O _{4}$ contains $Fe$ atoms of both $+2$ and $+3$ oxidation number.
It is a stoichiometric mixture of Ferrous $( FeO )$ and Ferric $\left( Fe _{2} O _{3}\right)$
oxides combined as $FeO$. $Fe _{2} O _{3}$.
Oxidation state of oxygen in $FeO$ and $Fe _{2} O _{3}$ is $-2$.
Let the oxidation state of $Fe$ in $FeO$ and $Fe _{2} O _{3}$ be $x$ and $y$ respectively.
As we know that the sum of the oxidation states of all the atoms or ions in a neutral compound is zero.
Therefore,
Oxidation state of $F$ e in $FeO -$
$x +(-2)=0$
$\Rightarrow x =2$
Oxidation state of $F$ in $Fe _{2} O _{3}-$
$2( y )+3(-2)=0$
$\Rightarrow 2 y =6 $
$\Rightarrow y =3$
Hence the oxidation number of $Fe$ in $Fe _{3} O _{4}$ are $+2$ and $+3$ respectively.