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Topic: from pH and molarity to Ka  (Read 5358 times)

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Offline sierrastudent

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from pH and molarity to Ka
« on: December 09, 2006, 10:02:21 PM »
The pH of a 0.0875 M Fe(NO3)3 solution is 1.68. Calculate Ka.
I know NO3 is a conjugate base of a strong acid, and therefore is neutral while Fe is a conjugate acid of a insoluble base. I use Fe for this equation.  Is the equation Fe+3 + H2O > Fe(OH)3 + H+?
« Last Edit: December 10, 2006, 05:41:35 AM by Borek »
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Offline Borek

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Re: from pH and molarity to Ka
« Reply #1 on: December 10, 2006, 05:49:06 AM »
You are close, although if you will balance the equation you will be closer. But I don't like this question - Fe(OH)3 is in general insoluble, so it is hard to speak about Ka, I would go personally for Kso.
« Last Edit: December 10, 2006, 06:21:34 AM by Borek »
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Offline burningq

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Re: from pH and molarity to Ka
« Reply #2 on: December 11, 2006, 12:22:53 AM »
I'm in sierrastudent's study group, and we worked on this again today.  What about this:

[H+] = 10-1.68 = 0.021

Fe3+ + 6H2O <--> [Fe(H2O)]3+
[Fe(H2O)]3+ + H2O <--> [Fe(H2O)]2+ + H3O+
   0.0875                              0                0
  -0.021                             +0.021         +0.021
---------------------------------------------------------------------
   0.0665                             0.021           0.021

Ka = (0.021)2/0.0665 = 0.0066
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Offline Borek

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Re: from pH and molarity to Ka
« Reply #3 on: December 11, 2006, 03:38:57 AM »
Quote from: burningq on December 11, 2006, 12:22:53 AM
Fe3+ + 6H2O <--> [Fe(H2O)]3+
[Fe(H2O)]3+ + H2O <--> [Fe(H2O)]2+ + H3O+

Sorry, it doesn't make any sense to me. None of the reactions is balanced.

Besides, this system is much more complicated - it contains numerous complex ions:
« Last Edit: December 11, 2006, 03:44:06 AM by Borek »
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Offline burningq

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Re: from pH and molarity to Ka
« Reply #4 on: December 11, 2006, 05:15:57 AM »
Sorry, tired and forgot a few subscripts:

[H+] = 10-1.68 = 0.021

Fe3+ + 6H2O <--> [Fe(H2O)6]3+
[Fe(H2O)6]3+ + H2O <--> [Fe(H2O)5OH]2+ + H3O+
   0.0875                                           0                0
  -0.021                                        +0.021         +0.021
---------------------------------------------------------------------
   0.0665                                        0.021           0.021

Ka = (0.021)2/0.0665 = 0.0066
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Offline Borek

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Re: from pH and molarity to Ka
« Reply #5 on: December 11, 2006, 06:16:45 AM »
Much better now :)

It all distills down to FeOH2+ then - and the reaction in question is

Fe3+ + H2O <-> FeOH2+ + H+

(I have removed everything that was there just to cloud the picture and confuse reader ;) )

You may assign some Ka value to this reaction, that's not incorrect approach.

It is just huge oversimplification, as there are many different species present in the solution. For example, almost 60% of Fe3+ is complexed with NO3- anions, thus your Ka value will use incorrect concentration of Fe3+, so it will be off.
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Offline burningq

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Re: from pH and molarity to Ka
« Reply #6 on: December 11, 2006, 06:44:50 AM »
 :) Thanks! Now to sleep...
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