Question

The oxidation number of $C$ in $HNC$ is:

Answer 1

The structure is $H-N\equiv C$
Oxidation number of $N$ is -1 because it is bonded to $H$ and $-2$ because of the pi and sigma bonds with $C$.
There is no contribution from the coordinate bond which is the third bond to $C$ in the structure, which gives us a total of $-3$.
Now let us suppose the oxidation number of ${}^{\prime }{C}^{\prime }$ is x:
$H-N\equiv C$
1+ (-3) + x = 0
x = +2