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• 1. pisgahch  |  May 12th, 2009 at 5:00 pm

  Predicting the oxidation number of a metal is easy provided that you know that the sum of the oxidation numbers is equal to the charge on the compound or ion and that the oxidation number of oxygen is assumed to be -2.
  Consider the general case where a and b are the subscripts, -c is the charge on the ion or molecule, and x is the oxidation number of manganese.
  ax .. -2b = -c Therefore ax + (-2b) = -c, solve for x
  x …. -2
  Mn(a)O(b)^-c
  x .. -8 = -1 Therefore x must be +7
  x .. -2
  MnO4-
  2x.. -6 = 0 Therefore x must be +3
  x … -2
  Mn2O3 ==> Mn
  x .. -4 = 0 Therefore x must be +4
  x .. -2
  MnO2

• 2. Niamh  |  May 12th, 2009 at 5:00 pm

  7
  3 (presuming thats a small two)
  4
  Edit..
  Whoops forgot the obvious
  You should look up the rules for finding the oxidation number if you have a chemistry book…
  But Oxygen is usually has an oxidation number of -2 (most cases)
  For each eqt you should end up with 0 unless there is a - or + after it (eg: in the first question)
  Ok so heres the first one.
  O4 = -2 x 4 = -8
  You want to end up with an answer of - 1 (beacause it is MnO4-)
  Therefore Mn must equal 7
  As - 8 + 7 = -1
  Sorry for the bad explaination I’ve just finished this chapter in school ;]

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