I'm stuck on determining the charge on various polyatomic ions according to the rule of charge balance. I keep getting 0 for each of these, but wonder if that's not the case.

  • $\ce{KMnO4}$

  • $\ce{K2C2O4}$

Can you please tell me the charge for these, and how you determined this?

  • 5
    $\begingroup$ Well, for starters, $\ce{KMnO4}$ is not an ion, just like $\ce{NaCl}$ is not an ion. The ions in $\ce{NaCl}$ are $\ce{Na+}$ and $\ce{Cl-}$, so the ions in $\ce{KMnO4}$ are... what? $\endgroup$ – orthocresol Sep 6 '16 at 5:05
  • $\begingroup$ I understand it's a compound. But what I'm being asked is "given the following formulas for these salts and using the rule of charge balance, determine the charge on the following polyatomic ions": KMnO4 $\endgroup$ – laroo Sep 6 '16 at 5:39
  • $\begingroup$ @orthocresol: you seem to be implying that it's just a simple matter of looking at the compound, looking at the ions within the compound, and reporting those back? $\endgroup$ – Monica Heddneck Sep 6 '16 at 7:36
  • $\begingroup$ @MonicaHeddneck Well, perhaps you don't know what the charge on the $\ce{MnO4}$ ion is. That's fine. However you probably should know the charge on a $\ce{K}$ ion, so maybe that lets you calculate the charge on the $\ce{MnO4}$ ion. $\endgroup$ – orthocresol Sep 6 '16 at 9:30
  • $\begingroup$ Yes, I've determined that the K has a +1 charge, and the O has a -8 charge. The K and O must add to equal 0, so the charge on Mn must equal +7. However, I'm being asked for the "charge on the polyatomic ions". Possible answers are A) 0; B) 1- C) 2- D)3- or E) 4- . I assume 0 can't be the answer for all of these ions (really, ionic compounds):but that's what it would be given the rule of charge balance. I guess I'm not sure what I'm being asked for-- $\endgroup$ – laroo Sep 6 '16 at 18:17

Apparantly, your question is phrased:

Given the following formulae for these salts and using the rule of charge balance, determine the charge on the following polyatomic ions: (examples)

That is a bad wording. Consider this better wording:

Given the following formulae of salts, determine which cations and anions exist and which are polyatomic. For the polyatomic ions only, determine their total charge.

Taking the potassium permanganate example $\ce{KMnO4}$:

  • You have potassium, manganese and oxygens which could (at first assumption) all exist as part of polyatomic ions.
  • However, deduce potassium from that above list. It does not form polyatomic ions.
  • Hence, whichever polyatomic ion may exist, it must counteract potassium’s charge.
  • Hence, the polyatomic ion is permanganate and displays a $-1$ charge to balance $\ce{K+}$. ($\ce{MnO4-}$)

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