Answer:
The oxidation state of chlorine in HClO4 is assumed to be unknown, suppose x. The oxidation state of hydrogen in this compound is +1 and the oxidation state of oxygen is -2.
The oxidation state of chlorine can be obtained applying the charge balance equation, as overall compound is neutral:
x + 1 + 4 × (-2) = 0
x - 7 = 0
x = 7
Thus, the oxidation state of chlorine is +7.
