Answer
+5
Work Step by Step
In a polyatomic ion, the sum of the oxidation numbers of all atoms is equal to the overall charge on the ion.
The overall charge on $NO_{3}^{-}$ ion is -1.
Let the oxidation number of nitrogen be $x$.
The oxidation number of each oxygen atom is -2.
Therefore, we can write
$x+3(-2)=-1$
Or $x=+5$
