1. Know the Oxidation States of Common Elements

* Oxygen (O): Usually has an oxidation state of -2, except in peroxides (like H2O2) where it's -1.

* Hydrogen (H): Usually has an oxidation state of +1, except in metal hydrides (like NaH) where it's -1.

2. Set up the Equation

The sum of the oxidation states of all atoms in a neutral compound must equal zero. Let 'x' represent the oxidation state of Cl:

x + 1 + 3(-2) = 0

3. Solve for x

x + 1 - 6 = 0

x = +5

Therefore, the oxidation state of Cl in HClO3 is +5.