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Topic: Amount of Positive Ions in Solution Question  (Read 3120 times)

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Offline Darren.S

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Amount of Positive Ions in Solution Question
« on: May 26, 2012, 10:06:49 PM »
Hi,
Please help me with the following question:

One mole of each of the following is dissolved in water. The solution containing the greatest amount of of positive ions is that of

a) glycerol, C3H5(OH)3
b) aluminium chloride, AlCl3
c) ammonia, NH3
d) methanoic acid, HCOOH


I'm not sure how to go about answering the question, each of the solutions seems to form positive ions with a 1:1 ratio. I'm tempted to think that it may be AlCl3 as the Cl- ions could react with any H+ ions in the water to form HCl which being a strong acid will dissociate to form the greatest concentration of H+ ions but I can't be sure.

Any input is greatly appreciated.
Thanks in advance 
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Offline Borek

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Re: Amount of Positive Ions in Solution Question
« Reply #1 on: May 27, 2012, 04:13:55 AM »
Can you write - for each substance - reaction equation, showing how the cations are produced?
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Offline Darren.S

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Re: Amount of Positive Ions in Solution Question
« Reply #2 on: May 27, 2012, 06:15:35 AM »
C3H5(OH)3  :rarrow:  C3H53+(aq) + 3OH-(aq)

AlCl3  :rarrow:  Al+(aq) + 3Cl-(aq)

NH3  :rarrow:  N3-(aq) + 3H+(aq)

HCOOH  :rarrow:  HCOO-(aq) + H+(aq)
HCOO-  :rarrow:  COO2-(aq) + H+(aq)

Is the answer ammonia?
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Offline Arkcon

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Re: Amount of Positive Ions in Solution Question
« Reply #3 on: May 27, 2012, 07:22:03 AM »
Is this one ...

Quote from: Darren.S on May 27, 2012, 06:15:35 AM
C3H5(OH)3  :rarrow:  C3H53+(aq) + 3OH-(aq)

Actually correct?  Does glycerol ionize at all?   Would say, sucrose, conduct electricity when dissolved in water?

Also ...
Quote

HCOOH  :rarrow:  HCOO-(aq) + H+(aq)
HCOO-  :rarrow:  COO2-(aq) + H+(aq)


Do we get lots of both reactions occurring?  Is methanolic acid a strong acid, like sulfuric?
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Offline Darren.S

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Re: Amount of Positive Ions in Solution Question
« Reply #4 on: May 27, 2012, 07:36:40 AM »
All right, if glycerol doesn't ionise in water then we can eliminate it from the question.
Also, the dissociation of Methanoic acid can be more correctly expressed as HCOOH(aq)  ::equil::  HCOO-(aq) + H+(aq) as it is a weak acid.

I am still unsure of the original question. Is the answer ammonia due to the production of 3 moles of H+ ions for every one mole of ammonia dissolved?   
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Offline Rutherford

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Re: Amount of Positive Ions in Solution Question
« Reply #5 on: May 27, 2012, 07:50:12 AM »
I think that it is AlCl3, because Al3+ creates with water [Al(H2O)6]3+ and then:
[Al(H2O)6]3++H2O :requil: [Al(H2O)5OH]2++H3O+
[Al(H2O)5OH]2++H2O ::equil:: [Al(H2O)4(OH)2]++H3O+
And again the same.

Ammonia will react like a base:
NH3+H2O ::equil:: NH4++OH- and the Kb=1.8*10-5.
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Offline Darren.S

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Re: Amount of Positive Ions in Solution Question
« Reply #6 on: May 27, 2012, 07:58:30 AM »
yes I originally thought that ammonia would form ammonium ions rather than dissociating to from N3- and H+.
By your reasoning I agree that it would be AlCl3, thank you for the answer.
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