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Topic: 9th problem IChO-analytical chemistry  (Read 9382 times)

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Offline Rutherford

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Re: 9th problem IChO-analytical chemistry
« Reply #15 on: March 01, 2014, 08:33:28 AM »
Yes.

How can this reaction: Pb(AcO)++AcO- :rarrow: Pb(AcO)2  be neglected?

Offline Radu

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Re: 9th problem IChO-analytical chemistry
« Reply #16 on: March 01, 2014, 11:23:18 AM »
  Yes, it is almost fully protonated, but, again, we don't care about the actual concentration of Pb2+. We know that the total concentration of lead in solution is s= 2.9*10-5=[Pb2+]+[Pb(AcO)]+] + [Pb(AcO)2] + [Pb(OH)+]. We explicit every concentration from β-s ,and make the approximations: [AcOH]≈1M and [H3O+]=cst=10-2.38.
 

Offline Radu

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Re: 9th problem IChO-analytical chemistry
« Reply #17 on: March 01, 2014, 11:29:41 AM »
  Raderford, β=[Pb(AcO)2]/ {[Pb2+][AcO-]2 = [Pb(AcO)2]*[H3O+]2/{[Pb2+]kaAcOH[AcOH]2.

Offline Rutherford

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Re: 9th problem IChO-analytical chemistry
« Reply #18 on: March 01, 2014, 11:57:28 AM »
I did it through the actual concentrations, it's pretty much the same, as I got the same answer as you.

I just don't know why should β(Pb(AcO)2) represent the cumulative constant and not the step-wise one. They should have marked this with β2 or β1, respectively. And how can the reaction of the PbAcO+ and AcO- be neglected?

Offline Radu

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Re: 9th problem IChO-analytical chemistry
« Reply #19 on: March 01, 2014, 12:16:15 PM »
Humm, I think it's not neglected, it is included in β2( the cumulative one), which is β1*k2, where k2 is the step-wise constant.
      I had this problem, too, this is why I got a wrong answer at the beginning, I had considered them as step-wise.

Offline Rutherford

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Re: 9th problem IChO-analytical chemistry
« Reply #20 on: March 01, 2014, 12:41:51 PM »
If it was step-wise there would be a little more calculation. I think that this is clear to me now. Thanks.

Offline Big-Daddy

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Re: 9th problem IChO-analytical chemistry
« Reply #21 on: March 02, 2014, 03:10:14 PM »
So how was it decided that the constant is cumulative, rather than step-wise?  ??? I can't tell from the question.

Inter-conversion is easy but we need to know whether the constant for which the value is given refers to the cumulative or step-wise equilibrium.

Offline Rutherford

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Re: 9th problem IChO-analytical chemistry
« Reply #22 on: March 03, 2014, 05:55:53 AM »
I asked the organizers. They said that it will be revised soon.

Offline Radu

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Re: 9th problem IChO-analytical chemistry
« Reply #23 on: March 03, 2014, 10:13:58 AM »
 I looked through and I found a book where they say there is a kind of convention:
   - k-s for step-wise stabilty constants, K-s for step-wise instability constants, β-s for cumulative stability, β-1 for instability cumulative constants.
    I know this is kind of foolish, because the inorganicians use opposite conventions, so they should have simply explicited them.

Offline Big-Daddy

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Re: 9th problem IChO-analytical chemistry
« Reply #24 on: March 03, 2014, 01:39:03 PM »
   - k-s for step-wise stabilty constants, K-s for step-wise instability constants, β-s for cumulative stability, β-1 for instability cumulative constants.

Sounds good, but I think that a) 'k' can't be used for any equilibrium constant per standard procedure, it refers to rate constants explicitly b) 'Kf' may be for step-wise stability constant but I don't think it would be for an instability constant.

Honestly I don't think instability constants are really symbolized in any way - it's just (stability constant)-1. It would make some sense if β was cumulative formation/stability, and Kf was step-wise formation/stability - I have seen that somewhere ...

Offline Rutherford

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Re: 9th problem IChO-analytical chemistry
« Reply #25 on: March 03, 2014, 02:01:53 PM »
I think that I read this from a IUPAC document (maybe the symbols were different, except β):
Mx++yL- :rarrow: [MLy]x-y βy=? where y represents the number of the ligands participating in the reaction. In our case, if y=1, we could understand it as a step-wise constant, but if y=2 then it would be cumulative. So β1(Pb(AcO)2) represents the constant of the reaction of one ligand and another molecule.

Offline Big-Daddy

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Re: 9th problem IChO-analytical chemistry
« Reply #26 on: March 03, 2014, 02:07:58 PM »
I think that I read this from a IUPAC document (maybe the symbols were different, except β):
Mx++yL- :rarrow: [MLy]x-y βy=? where y represents the number of the ligands participating in the reaction. In our case, if y=1, we could I see. So β1(Pb(AcO)2) refers to Pb(OAc)+ + AcO-  ::equil:: understand it as a step-wise constant, but if y=2 then it would be cumulative. So β1(Pb(AcO)2) represents the constant of the reaction of one ligand and another molecule.

Pb(OAc)2 whereas β2(Pb(AcO)2) refers to Pb2+ + 2AcO-  ::equil:: Pb(OAc)2. Thanks. I find Kf notation more common in IChO - Kf is definitely stepwise, e.g. Kf3 refers to binding from the form with 2 ligands to the form with 3 ligands.

Offline Rutherford

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Re: 9th problem IChO-analytical chemistry
« Reply #27 on: March 03, 2014, 03:12:14 PM »
But, after all, the organizers promised a revision soon, so we will see how they will formulate it then.

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