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Topic: Species Present at Equivalence Point??  (Read 9610 times)

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Offline remcalis

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Species Present at Equivalence Point??
« on: March 23, 2015, 11:05:49 PM »
I have a lab report due on Wednesday.  We did a titration of KHP with 0.1M NaOH. 

The question I am having trouble with is:
"What species were present in solution at the equivalence point of the titration?  Which of these species, if any, affected the pH of the solution at the equivalence point?  Write an equation that justifies your answer." So I believe the equation is as follows (but correct me if I'm wrong):

KHP(aq) + NaOH(aq) --> KNaP(aq) + H2O(l)
or, if leaving out the Na and the K:
HP- + OH- --> P2- +H2O

Now here is where I get stuck.  I am having trouble grasping how this actually works.  Am I correct in this thought process?:

At equivalence point, the only species present is P2-, because all of the HP- and OH- have reacted?
The species present at equivalence point are: K+, Na+, P2-, and H2O? But the only species that affected the pH of the solution at the equivalence point is P2-?  And as far as the "equation that justifies my answer":
P2-+ H2O --> HP- + OH-

Is all that correct? 

Thanks so much!

Thanks

Offline Borek

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Re: Species Present at Equivalence Point??
« Reply #1 on: March 24, 2015, 04:04:05 AM »
KHP(aq) + NaOH(aq) --> KNaP(aq) + H2O(l)
or, if leaving out the Na and the K:
HP- + OH- --> P2- +H2O

OK

Quote
At equivalence point, the only species present is P2-, because all of the HP- and OH- have reacted?

All species (H2P, HP- and P2-) are present. However, solution is dominated by P2-, as concentrations of others are many orders of magnitude lower.

Quote
The species present at equivalence point are: K+, Na+, P2-, and H2O? But the only species that affected the pH of the solution at the equivalence point is P2-?  And as far as the "equation that justifies my answer":
P2-+ H2O --> HP- + OH-

Looks OK.
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Offline remcalis

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Re: Species Present at Equivalence Point??
« Reply #2 on: March 24, 2015, 02:25:59 PM »
Thanks. I'm still having trouble understanding it though.  How does P2- affect the pH of the solution, specifically? I don't know why I'm having such a hard time with this acid/base stuff..

Offline magician4

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Re: Species Present at Equivalence Point??
« Reply #3 on: March 24, 2015, 04:49:40 PM »
ions (or ionic substances, to be more precise) can interact with water not only by being dissolved therein, but sometimes in the manner of acids / bases , additionally.

this also is true for all the ions related to phosphoric acid, where one of them ( phosphate PO43- or "P3-") would act as a base ( PO43- + H2:rarrow: HPO42- + OH- , pKb 1.68) exclusively, whereas the other two could act both ways:

"P2-" or hydrogenphosphate HPO42- ( pKa 12.32 ; pKb 6.79 )
PO43- + H3O+  :larrow: HPO42- + H2:rarrow: H2PO4- + OH-

"P-" or dihydrogenphosphate H2PO4- ( pKa 7.21 ; pKb 11.84 )
HPO42- + H3O+  :larrow: H2PO4- + H2:rarrow: H3PO4 + OH-

As you see, in P2- both these properties kinda "fight against each other". However, the base - properties being stronger by the factor of approx. 10 to the power of five, this effect will prevail, and therefore solutions of hydrogenphopsphate will end up being alkaline  ( pH approx. 9.77 , to be more specific)


regards

Ingo
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Offline Borek

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Re: Species Present at Equivalence Point??
« Reply #4 on: March 24, 2015, 08:09:26 PM »
How does P2- affect the pH of the solution, specifically?

Do you know Bronsted-Lowry theory of acids and bases?

ions related to phosphoric acid

Ingo, KHP most likely means potassium hydrogen phthalate.
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Offline magician4

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Re: Species Present at Equivalence Point??
« Reply #5 on: March 24, 2015, 08:53:22 PM »

Ingo, KHP most likely means potassium hydrogen phthalate.
so sorry, I completely misread that, then
(an excuse for me might be that the abbreviation "P" (  esp. in biochemistry) keeps showing up again and again, meaning shorthand for "phosphate(-esters etc.)" )

 :rarrow:will do better next time

again, thank you for your input

regards

Ingo
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