Oxidation states (1 Viewer)

[nahi11]

Hi,

I got this question from HSC 2009 multiple choice paper. What is the oxidation state of chromium and how would I find it? Looking at an electron diagram of Cr I see it has one valence electron, so it would have a oxidation state of one? Looking on the internet it says the most common are 2+ and 3+, how am I supposed to find this out?

The answer is C

9
One test used for random breath testing in NSW involved crystals of potassium
dichromate reacting with ethanol. In this reaction the orange dichromate ion, Cr2O72−,
changes to the green chromium ion, Cr 3+.
Which statement is true for this reaction?
(A) Chromium has lost electrons and reached a lower oxidation state.
(B) Chromium has lost electrons and reached a higher oxidation state.
(C) Chromium has gained electrons and reached a lower oxidation state.
(D) Chromium has gained electrons and reached a higher oxidation state.

Thanks for your time

 

[nahi11]

I think I figured it out, but would still welcome any explanations.

I knew that the total oxidation state of the molecule is -2.
So using equations:

2Cr + 7(-2) =-2
Cr=+6

Is this method valid?

 

[nahi11]

Got it thanks