pH/Titration Question


by woox
Tags: ph or titration
woox
woox is offline
#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?
Phys.Org News Partner Science news on Phys.org
Internet co-creator Cerf debunks 'myth' that US runs it
Astronomical forensics uncover planetary disks in Hubble archive
Solar-powered two-seat Sunseeker airplane has progress report
Borek
Borek is offline
#2
May10-09, 05:39 PM
Admin
Borek's Avatar
P: 22,714
acid base titration equivalence point calculation

Note: you should not use Henderson-Hasselbalch, however, you should not forget about dilution.
woox
woox is offline
#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
Borek is offline
#4
May10-09, 06:04 PM
Admin
Borek's Avatar
P: 22,714

pH/Titration Question


So show details of your work, hard to say what you did wrong not knowing what you did.
woox
woox is offline
#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
Borek is offline
#6
May11-09, 02:36 AM
Admin
Borek's Avatar
P: 22,714
Please reread my very first post in this thread. Every word of it.
woox
woox is offline
#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
Borek is offline
#8
May11-09, 11:28 AM
Admin
Borek's Avatar
P: 22,714
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.


Register to reply

Related Discussions
Titration question Biology, Chemistry & Other Homework 1
electrical conductivity of solutions and titration curve question Biology, Chemistry & Other Homework 0
Titration Question...please help. Biology, Chemistry & Other Homework 3
Titration Calculation question. Chemistry 1
titration question Chemistry 1