A few oxidation state questions

[cyrano]

These are from the ACS Chemistry Exam review book. Thanks a lot in advance!

1) In which compound does vanadium have the lowest oxidation state?
A) V2O5
B) V2O3
C) VO2
D) VO

Answer: C

I think it's A and can't understand why it isn't.

2) What is the oxidation mumber of chromium in Na2Cr2O7?
A) +12
B) +6
C) +3
D) -2

Answer: C

I think it's B on this one and can't understand why I'm wrong.

Once again I'd really appreciate and explanation!

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[klutzy1987]

I am pretty sure that both questions are wrong. For the first question the answer should be D because the oxidation state there is a +2 while in C it is a plus 4.

The second question, you are correct, the answer is B, +6, The 7 Oxygens give a -14 and the 2 NA make a +2 so the net is -12. Divide by 2 gives -6, so each Chromium needs to carry a +6 charge to offset the -6.

 

[cyrano]

Thanks a lot! I think you're right. For the first one for some reason I answered with A but that one was the highest oxidation state. Oops.