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Topic: Calculating valency of Bromine in Sodium bromate (NaBrO3)  (Read 14753 times)

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Offline nasno

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Calculating valency of Bromine in Sodium bromate (NaBrO3)
« on: October 27, 2011, 08:24:06 AM »
Hi guys,

Sorry, i'm having a dumb moment and can't seem to get my head around a question/answer, could someone point me in the right direction please?

Q. Calculate the valence of bromine in the following compounds.  Is bromine a cation or an anion? 

Sodium bromate NaBrO3

A. Na+ Br+5 O3-6  ∴ Br +5 is a cation

that's my answer but I'm wrong as the answer is given as Br +5 is a complex anion.

What am not understanding here?

thanks guys.

Offline fledarmus

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #1 on: October 27, 2011, 08:34:56 AM »
Bromate is a complex anion - was this a misprint in the test? Or (and please don't take offense) did you misread the question?

If you actually look at the structure of the bromate anion, although the bromine atom does have a formal charge of +5, it really isn't either a cation or an anion. It is covalently bound to the oxygen atoms, with double bonds to two of them, and a single bond to a third. The anion is on the oxygen atom.

Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #2 on: October 27, 2011, 08:46:51 AM »
sorry, the full question copied and pasted is;

Calculate the valence of bromine and nitrogen in the following compounds.  In each case, is bromine or nitrogen a cation or an anion?  

•   Sodium Bromide (NaBr) Na+ Br- (Bromine -1 anion)
•   Bromine chloride (BrCl) Br+ Cl- (Bromine +1 cation)
•   Hypobromous acid (HOBr) H+ O-2 Br+ (Bromine +1 cation)
•   Sodium bromate (NaBrO3) Na+ Br+5 O3-6 (Bromine +5 cation)
•   Ammonium chloride (NH4Cl) N-3 H4+4 Cl- (Nitrogen -3 anion)
•   Nitrogen gas (N2) non-ionized Gas
•   Sodium nitrite (NaNO2) Na+ N+3 O2-4 (Nitrogen +3 cation)
•   Nitric Acid (HNO3) H+ N+5 O3-6 (Nitrogen +5 cation)

those in bold above are my answers.
below are the answers I've been given to check my results

•   Sodium Bromide (NaBr) Bromine -1 anion
•   Bromine chloride (BrCl) Bromine +1 cation
•   Hypobromous acid (HOBr) Bromine +1 complex anion
•   Sodium bromate (NaBrO3) Bromine +5 complex anion
•   Ammonium chloride (NH4Cl) Nitrogen -3 cation
•   Nitrogen gas (N2) non-ionized Gas
•   Sodium nitrite (NaNO2) Nitrogen +3 complex anion
•   Nitric Acid (HNO3) Nitrogen +5 complex anion

I'm embarrassed to say I'm a little confused  ???

Offline Borek

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #3 on: October 27, 2011, 09:54:02 AM »
This is a horrendous question. As fledarmus wrote, you can't treat a single atom in a large, covalently bonded molecule, as being a cation or an anion.

Could be it is just a lousy wording, and what the question really asks about is whether Br/N are parts of anion or cation. Take a look at the ammonium chloride answer.
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Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #4 on: October 27, 2011, 10:26:16 AM »
yikes. maybe context may help?
It's from a competency course for a new job, for context it's water treatment and the compounds are what we'll encounter in solution.
I'm assuming the question means for me to work out what charge the ions would have in solution so we can predict what reactions may occur with other compounds in solution.
Does that help?
The answer sheet came from my supervisor, but I'm starting to question that now (maybe they're trying to trick me? :) ). I thought I was more or less on the right path before I checked against them.

re: NH4CL, is this correct then?
Ammonium chloride (NH4Cl) N+5 H4+4 Cl-7 (Nitrogen +5 cation)

Thanks for your help guys.
« Last Edit: October 27, 2011, 10:39:32 AM by nasno »

Offline fledarmus

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #5 on: October 27, 2011, 10:49:18 AM »
In water treatment, although the question is poorly worded, the context makes sense in light of Borek's answer. If you are trying to remove salts from water, you usually use ion exchange columns or other forms of ion exchanging technology. If you analyze for bromine and need to remove it, distinguishing whether it is part of an anion or cation will determine what type of ion you try to exchange it with.

Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #6 on: October 27, 2011, 11:00:06 AM »
sorry guys, still a little confused.  :-[
are either sets of answers right or are they both wrong?
I'm not really sure what Borek meant by what the question is really asking. are Br/N parts of an anion/cation compound as a whole..? i thought it asked if Br or N act as cation or anion in forming the compound.

edit: re-read your post fledarmus, yes, i believe that's the point of the question, but I'm confused why the supplied answers list positive charges as anions and vice versa. i'm dumbfounded.

Offline fledarmus

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #7 on: October 27, 2011, 11:22:46 AM »
sorry guys, still a little confused.  :-[
are either sets of answers right or are they both wrong?
I'm not really sure what Borek meant by what the question is really asking. are Br/N parts of an anion/cation compound as a whole..? i thought it asked if Br or N act as cation or anion in forming the compound.

edit: re-read your post fledarmus, yes, i believe that's the point of the question, but I'm confused why the supplied answers list positive charges as anions and vice versa. i'm dumbfounded.

Yes, that's what makes it such a terribly worded question. I believe the answer they are looking for in English would be, "in NaBrO3, bromine has a valence of +5 and is part of a complex anion". Unfortunately when the question and answers are matched in the manner you have presented, you almost have to be able to read the mind of the person who prepared the test to give them the answer they are looking for.

Offline Borek

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #8 on: October 27, 2011, 12:52:47 PM »
re: NH4CL, is this correct then?
Ammonium chloride (NH4Cl) N+5 H4+4 Cl-7 (Nitrogen +5 cation)

No, that's completely off, regardless of what they look for.

Ammonium chloride (NH4Cl) Nitrogen -3 cation

Ammonium chloride is a salt that dissociates into NH4+ cation and Cl-. Note that answer given lists -3 for nitrogen but also lists it as a cation. Ion with -3 charge is an anion, so the answer contains contradicting information. However, this answer would be correct if the question was about what is the formal charge of the nitrogen, and about whether nitrogen is part of the cation or the anion.

All other answers fit this approach. It doesn't make it correct.
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Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #9 on: October 27, 2011, 09:05:19 PM »
re: NH4CL, is this correct then?
Ammonium chloride (NH4Cl) N+5 H4+4 Cl-7 (Nitrogen +5 cation)

No, that's completely off, regardless of what they look for.
Thank %#*& for that, I won't tell you where i got that from but it wasn't my working and it didn't make any sense to me.

Ok, I think i understand. I'm going to go back and ask for some clarification on what the question is really asking for.

The rationale behind my answers was to work out the individual charge of elements in the compound and note whether that element initially was an anion/cathion (trying to answer the wording of the question).
I would think what they are really asking is what the compounds would dissociate to in solution and the charge they would carry as an ion in solution, but I'm not sure that the questions' wording is asking that.
But in context, it's what makes sense to me to be asking.

Sorry guys, thanks for your patience, it's been 15yrs since i've had to use this stuff and i'm obviously quite rusty and the question is confusing me and my memory.

cheers for you help so far, glad I found this place.

Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #10 on: October 27, 2011, 09:55:44 PM »
My revised answer with the belief that the question referred to dissociation in solution.

Q. Calculate the valence of bromine and nitrogen in the following compounds.  In each case, is bromine or nitrogen (forming a, or part of) a cation or an anion (while in solution)?  

•   Sodium Bromide (NaBr) Na++ Br- (∴ forming an anion in solution)
•   Bromine chloride (BrCl) Br++ Cl- (∴ forming a cation in solution)
•   Hypobromous acid (HOBr) H++ OBr- (∴ forming an anion in solution)
•   Sodium bromate (NaBrO3) Na++ BrO3- (∴ forming an anion in solution)
•   Ammonium chloride (NH4Cl) NH4++ Cl- (∴ forming a cation in solution)
•   Nitrogen gas (N2) non-ionized dissolved gas
•   Sodium nitrite (NaNO2) Na++ NO2-(∴ forming an anion in solution)
•   Nitric Acid (HNO3) H++ NO3-(∴ forming an anion in solution)

Am I on the right track now?

Offline fledarmus

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #11 on: October 27, 2011, 10:32:20 PM »
I like that question - and those answers - a lot more! Although I notice you didn't actually put in the valence of the bromine and nitrogen again  ;)

Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #12 on: October 27, 2011, 10:41:15 PM »
Q. Calculate the valence of bromine and nitrogen in the following compounds.  In each case, is bromine or nitrogen (forming a, or part of) a cation or an anion (while in solution)?  

•   Sodium Bromide (NaBr) Na++ Br- (∴ forming an anion in solution, Br -1 valence)
•   Bromine chloride (BrCl) Br++ Cl- (∴ forming a cation in solution, Br +1 valence)
•   Hypobromous acid (HOBr) H++ OBr- (∴ forming an anion in solution, Br +1 valence)
•   Sodium bromate (NaBrO3) Na++ BrO3- (∴ forming an anion in solution, Br +5 valence)
•   Ammonium chloride (NH4Cl) NH4++ Cl- (∴ forming a cation in solution, N -3 valence)
•   Nitrogen gas (N2) non-ionized dissolved gas
•   Sodium nitrite (NaNO2) Na++ NO2-(∴ forming an anion in solution, N +3 valence)
•   Nitric Acid (HNO3) H++ NO3-(∴ forming an anion in solution, N +5 valence)

opps  :D
I hope that's right, it all sits well in my head.

edit: is it important, or should I note that it is a complex anion/cation?
« Last Edit: October 27, 2011, 11:04:32 PM by nasno »

Offline Borek

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #13 on: October 28, 2011, 04:18:23 AM »
Bromine chloride (BrCl) Br++ Cl- (∴ forming a cation in solution, Br +1 valence)

I am not sure about chemistry of BrCl, but Br+ in solution doesn't look convincing. If anything I would expect reaction with water, with Cl-/Br- and OCl-/OBr- between products.
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Offline nasno

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Re: Calculating valency of Bromine in Sodium bromate (NaBrO3)
« Reply #14 on: October 28, 2011, 11:39:15 PM »
yes, I think that will be a transition state as it moves between HOBr, OBr-, H+ and HCl with pH and temperature determining which is most prevelant.

Thanks for all your help guys, forums are only ever as good as their members and this has been a huge help. ;D

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