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MCAT Chemistry Question — Oxidation

  • by John
  • Mar 07, 2017
  • MCAT Question of the Day

What is the oxidation state of chromium in the dichromate ion, Cr2O72-?

 

  1. -2
  2. 0
  3. +6
  4. +12
Click for Explanation
The rules for oxidation states are as follows:

  1. In a compound, oxidation states of all elements sum to the net charge on the ion or neutral compound. Free elements have an oxidation number of 0.
  2. The more electronegative element has a negative oxidation state, fluorine and oxygen being the most and second most electronegative.
  3. Group IA elements are +1, Group IIA elements are +2, Group VIIA elements are usually -1 unless bonded to a more electronegative element, oxygen usually -2, hydrogen usually +1, fluorine always -1, chlorine usually -1.

 

In this case, the oxidation states of all elements sums to -2. 7 oxygens have an oxidation state of -2, summing to -14. The 2 chromiums must sum to +12, giving an oxidation state of +6 for each chromium.

  1. -2, incorrect, This is the overall oxidation state.
  2. 0, incorrect, This answer indicates that 7 oxygens have a total oxidation state of -2.
  3. +6, correct.
  4. +12, incorrect, This answer neglects the fact that there are 2 chromiums, therefore the +12 must be divided by 2.

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