In which of the following compounds does chlorine show its highest oxidation state?

The correct answer is HClO4.

• Chlorine shows its highest oxidation state in HClO4.
• Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that the electron pair in a covalent bond belongs entirely to a more electronegative element.
• Now let us one by one find out the oxidation state/number of Chlorine in each of its compounds:
  • HCl - Here oxidation number of hydrogen is +1 and the oxidation number of chlorine is -1. Since the algebraic sum of the oxidation number of all atoms in a compound must be zero. 
  • Ca(OCl)2 - Ca has +2 oxidation state and the two OCl ions have -1 oxidation state each. As we know, oxygen depicts -2 oxidation state, and the ion has overall -1 charge, the oxidation state of Chlorine is +1. (since -2 + x = -1 ⇒ x = +1).
  • HClO4 - Hydrogen has +1 oxidation state and oxygen has -2 oxidation state on each atom. So, we can calculate the oxidation state of Chlorine by this method : +1 + x + (-8) = 0 ⇒ x = +7. (since the algebraic sum of the oxidation number of all atoms in a compound must be zero).
  • KClO3 - Potassium has +1 oxidation state and oxygen has -2 oxidation state on each atom. So, we can calculate the oxidation state of Chlorine by this method : +1 + x + (-6) = 0 ⇒ x = +5. (since the algebraic sum of the oxidation number of all atoms in a compound must be zero).
  • Thus, we conclude that Chlorine shows its highest oxidation state of +7 in HClO4.