Why do I get pH 4.98 instead of 5.12 for NH3-HCl titration at equivalence point?

[woox]

Homework Statement

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 can't figure out why. I would assume you could use the henderson equation but I only get 4.98 which is wrong.

Any help?

 

[Borek]

acid base titration equivalence point calculation

Note: you should not use Henderson-Hasselbalch, however, you should not forget about dilution.

 

[woox]

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]

So show details of your work, hard to say what you did wrong not knowing what you did.

 

[woox]

So show details of your work, hard to say what you did wrong not knowing what you did.

k... using ICE table below:

 

[Borek]

Please reread my very first post in this thread. Every word of it.

 

[woox]

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]

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.