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Chromium ions in water

  1. Jan 11, 2013 #1
    Cr3++?OH-→ Cr(OH)3

    I think it should be 9 but the mark scheme says 3 but accept correct multiples because the 3+ charge has to be cancelled by 3- and this has to produce 3 O
  2. jcsd
  3. Jan 12, 2013 #2
    What are you wanting to know? ...Are you wondering about the coefficient in front of the OH?
    Because that is 3.

    Each OH- has a charge of -1 (it's one of those polyatomic ions).
    So the Cr+3 and 3 OH- ions cancel each other out and is neutral overall.

    I hope I answered what you wanted to know...
    Sorry if I didn't realize you were asking something else!
  4. Jan 12, 2013 #3
    I still think its dodgy because you are producing 3 OH so need 9 like in electrolysis, where 2O2-→4e- + O2 on the principle of that it should be 9 but i'm thinking the question itself is incorrect because it should be 3Cr3++9OH-→Cr(OH)3
  5. Jan 12, 2013 #4
    You just have to make sure there is the same number of elements on each side.

    3Cr+3 + 9OH-→Cr(OH)3

    This isn't balanced because there are 3 Cr's on the left, and only 1 Cr on the right.
    And on the left, there's 9 OH's, but on the right, there's only 3 OH's.

    To balance this, you could add a 3 coefficient to the right side.

    Then you'd have:

    3Cr + 9OH --> 3Cr(OH)3

    Now there are 3 Cr's on each side, and 9 OH's on each side.

    *But this can be simplified further.
    Just divide each coefficient by 3, and you'd get the correct formula:

    Cr + 3OH --> Cr(OH)3
  6. Jan 12, 2013 #5


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    Staff: Mentor

    You are both inconsistent in treating charges - please always list them, as they are as important as atoms are. Reaction equation needs to have both atoms and charge balanced - atoms reflect mass conservation, charge reflects charge conservation
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