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Homework Help: Oxidation numbers

  1. Nov 7, 2017 #1
    1. The problem statement, all variables and given/known data
    Identify the element oxidized and the element reduced, in this chemical equation:
    H3AsO4+H2S ---> H3AsO3+S+H2O

    2. Relevant equations

    3. The attempt at a solution

    So I have learned the rules for oxidation numbers:

    1. The oxidation number of an element is always 0 regardless of its subscript. This is true as long as they are not part of compound.
    2. The oxidation number of a simple ion is the charge of the ion.
    3.The oxidation number of oxygen in compounds is -2.
    4. The oxidation number of hydrogen in compounds is +1.
    5.Group 1 and group 2 metals always have a +1, and +2 value, respectively, in compounds.
    6. All other oxidation numbers are assigned so that the net sum of the oxidation numbers equals the net charge of the molecule or complex ion.

    So with what I know I can figure out the oxidation numbers for the oxygens and hydrogens in the chemical equation but how do I figure out the numbers for arsenic and sulfur?
  2. jcsd
  3. Nov 7, 2017 #2


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    Science Advisor

    So let's start with H2S. It has no net charge, so you can apply rules 6 and 4.
  4. Nov 7, 2017 #3
    H2 would be +1 but I'm not sure what S would be. Would it be -2 so that it equals +1 overall?
  5. Nov 7, 2017 #4


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    Staff: Mentor

    No, H2 would be not +1. You have two H at +1 each, so it is +2 total for hydrogen. If so, what must be the charge on the sulfur atom so that the molecule is neutral?
  6. Nov 7, 2017 #5
    Oh I understand. Thats why S is -2.
  7. Nov 7, 2017 #6


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    Staff: Mentor

    That's it.
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