What is the oxidation state of chlorine in KClO3?

##+5##
Excellent question! I love oxidation state questions.
First let’s look at the ##K## (potassium). In an ionic compound the potassium’s oxidation state is always ##+1##.
Next let’s look at the chlorate ##ClO_3^-##. The charge on the polyatomic ion is ##-1##. You may be asking yourself how do I know its charge will be ##-1##?
Since the entire compound ##KClO_3## has a charge of ##0## and the ##K## will have a charge of ##+1## the ##ClO_3## must balance the ##K##’s ##+1## in the form of ##-1## for a net charge of ##0##.
When oxygen is with another element that is less electronegative than it is the charge on the oxygen is ##-2##. There are ##3## oxygen atoms in the chlorate ion for a total of ##-6## charge on the total of the ##3## oxygen atoms.
Thus ##charge of Cl## ##+(-6)=-1##. That means that the charge on chlorine in potassium chlorate is ##+5##.
Have a great day!!