Answer
a) In $ClO^{-}$ ion, oxidation state of $Cl = +1$
b) In $ClO_{2}^{-}$ ion, oxidation state of $Cl = +3$
c) In $ClO_{3}^{-}$ ion, oxidation state of $Cl = +5$
d) In $ClO_{4}^{-}$ ion, oxidation state of $Cl = +7$
Work Step by Step
For polyatomic ion, algebraic sum of oxidation state will be equal to the charge of that ion.
a) In $ClO^{-}$, oxidation state of O = -2. Let's say oxidation state of Cl = $x$ Hence, $x + (-2) = -1$ by solving we get $x = +1$
So, oxidation state of $Cl = +1$ here.
b) In $ClO_{2}^{-}$, oxidation state of O = -2. Let's say oxidation state of Cl = $x$ Hence, $x + (-2)\times2 = -1$ by solving we get $x = +3$
So, oxidation state of $Cl = +3$ here.
c) In $ClO_{3}^{-}$, oxidation state of O = -2. Let's say oxidation state of Cl = $x$ Hence, $x + (-2)\times3 = -1$ by solving we get $x = +5$
So, oxidation state of $Cl = +5$ here.
d) In $ClO_{4}^{-}$, oxidation state of O = -2. Let's say oxidation state of Cl = $x$ Hence, $x + (-2)\times4 = -1$ by solving we get $x = +7$
So, oxidation state of $Cl = +7$ here.
