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Se oxidation state!

  1. Jan 12, 2004 #1
    Redox question:

    12.53 ml of .05093 M selenium oxide (SeO2) reacted exactly with 25.52 ml of .1 M CrSO4. Cr+3 is formed. To what oxidation state is Selenium converted?

    I think that the ml + M stuff gives you the ratio between moles of SeO2 and moles of CrSO4 (1m SeO2 = 4m CrSO4). But how am I supposed to figure out what happens to the Se?
  2. jcsd
  3. Jan 13, 2004 #2
    Okay, this is how I'm looking at it, based solely on the information alone and not by looking up the oxidation states of selenium. I don't know if it's right but...
    Ratio is 1:4, we know that, and we also know that chromium goes from +2 to +3. So SeO2 needed 4 moles of CrSO4 and so it follows that it needed 4 moles of e- from the Cr+2. So one SeO2 recieves 4 e-'s and makes the reduction: Se(+4) + 4e- --> Se(0) which is the selenium metal.

    Wow it looks so nice, we've solved the problem, and I don't know if it's right. Something came up though, when you write out the equation there's a problem with the sulfate ion floating around:

    SeO2 + CrSO4 --> Se + Cr2O3 + SO4(-2)

    so maybe it reacts with the water in the solution (assuming everything's aqueous and it probably is), and forms H2SO4.

    2H+ + SeO2 + 4CrSO4 --> Se + 2Cr2O3 + H2SO4

    I don't think this is right, so can someone please help.
  4. Jan 18, 2004 #3
    someone care to help? I'm sorry but I just really want to know the answer, it bugs me when I can't solve a problem.
  5. Jan 18, 2004 #4


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    I'm thinking its 2+, that is it has been reduced while the chromium has been oxidized. Nonmetalic elements usually have oxidation states at even intervals e.g. from 4+ to 2+. Tis an oxidation reduction reaction.
  6. Jan 18, 2004 #5
    Se has oxidation states of +6, 4, 1, and -2. +1 being very rare. http://www.ivstandards.net/extras/pertable/
    Regardless though, based on the information that was given and my reasoning, was it a correct approach?
    And selenium sulfate doesn't exist, I can't find it on chemfinder, so I don't think that that will even form. SO what the hell is going on...
  7. Jan 19, 2004 #6


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    Chromium is oxidized, which means it has lost some electrons. Therefore it is a reducing agent; something has been reduced. Selenium is reduced. If the original assumption was right-1 Se for every 4 Cr, than we can assume that 4 electrons are lost for every 1 Se. Assuming that the original oxidation state of Se was 4+ we can say that the oxidation state for Se is now 0. However, I am not completely sure about this. I'll check back later with other details, I am a bit busy right now.
  8. Jan 20, 2004 #7


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    Yeah, the above is correct.
  9. Jan 21, 2004 #8
    what happens to SO4(2-)?
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