The oxidation number of chlorine in Ca(ClO3)2 is:
Correct answer: C. +5
- A. -1
- B. +3
- C. +5
- D. -6
Explanation
In the compound Ca(ClO3)2, calcium has an oxidation state of +2, being a Group 2 metal. Each ClO3- ion must balance to -1 overall charge. Oxygen typically has an oxidation state of -2. Since there are three oxygen atoms, their total contribution is -6. To balance this to -1 for each ClO3-, chlorine must have an oxidation state of +5. Therefore, the oxidation number of chlorine in Ca(ClO3)2 is +5.Options -1 and -6 are incorrect as they do not balance the charges correctly. Option +3 does not account for the full negative contribution of the oxygen atoms.
This question appeared on the ETEA MDCAT 2008 paper, which you can sit online with every answer explained.
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Oxidation involves loss of electrons, increase in oxidation number or addition of oxygen, while reduction involves electron gain, decrease in oxidation number or removal of oxygen. The topic covers oxidising and reducing agents, oxidation-number changes, redox equations and the distinction between oxidation at the anode and reduction at the cathode.
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