Attachment data exported to attachments-and-meta.csv. MCAT Chemistry Question -- Oxidation | Blueprint Prep MCAT
Return to MCAT® Blog Homepage

MCAT Chemistry Question — Oxidation

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.

Want more MCAT practice?

We’ve got options for every schedule and learning style!

From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!

Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT expert using the form below. No obligation, just expert advice.

Create your Free Account to access our MCAT Flashcards SIGN UP NOW
MCAT is a registered trademark of the Association of American Medical Colleges (AAMC), which is not affiliated with Blueprint.