Question

The oxidation state of $Fe$ in $\mathop K\nolimits_4 \left[ {\mathop {Fe(CN)}\nolimits_6 } \right]$ is:

Answer 1

The first nitrosyl complex was discovered by J Priestley in 1790, $\mathop K\nolimits_4 \left[ {\mathop {Fe(CN)}\nolimits_6 } \right]$ . Here, we first write the oxidation state of each molecule individually leaving only Fe.

Complete step by step answer:
The oxidation state is the number of electrons that are lost, gained, or shared by a certain atom with other atoms in the same molecule
Here, we will focus on all the fixed components and then work out the oxidation state of $Fe$.
The net charge on the molecule is 0. Therefore, all the oxidation states will add up to 0 after they are multiplied by the number of molecules.
$(4 \times {\text{oxidation state of }}K) + ({\text{oxidation state of Fe}}) + (6 \times {\text{oxidation state of CN}}) = 0$
Let the oxidation state of $Fe$ be $x$
We know that almost all elements from group 1 have an oxidation state of $+ 1$. This rule never changes. Thus, we will assume that the oxidation state of $K$ is $+ 1$. And the oxidation state of $CN$ is $- 1$.
Now plugging in the numbers in the equation given above, we get,
$[4 \times ( + 1)] + [x] + [6 \times ( - 1)] = 0$
$\Rightarrow + 4 + x - 6 = 0$
$\Rightarrow x - 2 = 0$
$\therefore x = + 2$
So, the oxidation state of $Fe$ is $+ 2$..
Thus, the oxidation state of $Fe$ is +2 when the oxidation states of $K$ and $CN$ are $+ 1$ and $- 1$respectively.

Note:
Always check what the net charge on the given ion or molecule is, this will change the number present on the R.H.S. of the equation.
For example: If we consider $\mathop {SO}\nolimits_4^{2 - }$ then to calculate the oxidation state of $S$, we need to consider the equation.
$({\text{oxidation state of S}}) + (4 \times {\text{oxidation state of O}}) = - 2$
Let the oxidation state of $S$ be $x$
Then, $[x] + [4 \times ( - 2)] = - 2$
$\Rightarrow x + ( - 8) = - 2$
$\Rightarrow x = + 6$
Here the oxidation state of S will be +6. If we do not pay attention to the net charge, the answer will turn out to be +8, which is not possible.