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PH/Titration Question

by woox
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woox
#1
May10-09, 05:20 PM
P: 17
1. The problem statement, all variables and given/known data
Calculate the pH at the equivalence point for the titration of 0.20 M NH3 with 0.20 M HCl? Kb of ammonia is 1.8e-5

This is just a old test I am going over, I managed to get a pH of 4.98. but that is not right the right answer is 5.12 and I cant figure out why. I would assume you could use the henderson equation but I only get 4.98 which is wrong.

Any help?
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Borek
#2
May10-09, 05:39 PM
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acid base titration equivalence point calculation

Note: you should not use Henderson-Hasselbalch, however, you should not forget about dilution.
woox
#3
May10-09, 05:44 PM
P: 17
Quote Quote by Borek View Post
acid base titration equivalence point calculation

Note: you should not use Henderson-Hasselbalch, however, you should not forget about dilution.
Even if I did not use HH equation I am getting the wrong answer.

Borek
#4
May10-09, 06:04 PM
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PH/Titration Question

So show details of your work, hard to say what you did wrong not knowing what you did.
woox
#5
May10-09, 07:09 PM
P: 17
Quote Quote by Borek View Post
So show details of your work, hard to say what you did wrong not knowing what you did.
k.... using ICE table below:

Borek
#6
May11-09, 02:36 AM
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Please reread my very first post in this thread. Every word of it.
woox
#7
May11-09, 10:20 AM
P: 17
Quote Quote by Borek View Post
Please reread my very first post in this thread. Every word of it.
I did and didn not use the HH eq, not do i understand what you mean by dilution.

I also read link, and this is onlything really useful that I understood but still does not explain what I am doing wrong:
Spoiler
In the case of titration of weak base with strong acid, situation is very similar - pH at the equivalence point is determined by the weak base salt hydrolysis. Thus we need pKa of conjugated acid to calculate H+ and pH. Check lecture and cheat sheet mentioned above for details.


It would be great if you told me what I am doing wrong...
Borek
#8
May11-09, 11:28 AM
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Try here: dilution effects. Generally speaking, you have mixed two solutions, concentrations have changed as the final volume differs from the initial for each reagent.


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