# Chromium ions in water

1. Jan 11, 2013

### Dalek1099

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. Jan 12, 2013

### 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!

3. Jan 12, 2013

### Dalek1099

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

4. Jan 12, 2013

### Lo.Lee.Ta.

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

5. Jan 12, 2013

### 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