Chromium ions in water

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

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 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!

 Quote by Lo.Lee.Ta. 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!
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

Chromium ions in water

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

 Admin 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