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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: XeLa. on November 02, 2017, 08:58:13 PM

Title: Weak acid and weak base mixture
Post by: XeLa. on November 02, 2017, 08:58:13 PM
I was trying to calculate the pH of a mixture containing 0.35 M NH3 and 0.20 M acetic acid. I first considered that acetic acid reacted with ammonia giving acetate ion and ammonium ion. Since acetic acid is the limiting reagent, at this stage of calculation we would have 0.15 M NH3, 0.2 M NH4+, and 0.2 M CH3COO-.

I then preceded to consider the pH of the ammonia-ammonium ion buffer (since this is now the strongest acid-base pair) using Henderson-Hasselbalch equation and found the pH of the solution (using pOH from H-H equation) to be 9.13.

Now, I went to check my solution using an online calculator and found the pH of the solution to be roughly 9.10 instead. Why would this be the case? Would I have to consider the hydrolysis by acetate ions as well?

Any help would be appreciated!
Title: Re: Weak acid and weak base mixture
Post by: Borek on November 03, 2017, 03:53:36 AM
In my experience 9.10 and 9.13 is exactly the same answer (and as the ionic strength of the solution is 0.2 both are wrong, in reality pH will be a bit higher, something like 9.3 if I were to trust my calculators).

First thing to check is the dissociations constants used.
Title: Re: Weak acid and weak base mixture
Post by: XeLa. on November 03, 2017, 07:39:11 AM
Quote from: Borek on November 03, 2017, 03:53:36 AM
In my experience 9.10 and 9.13 is exactly the same answer (and as the ionic strength of the solution is 0.2 both are wrong, in reality pH will be a bit higher, something like 9.3 if I were to trust my calculators).

First thing to check is the dissociations constants used.

Okay, thanks, Borek! :)

I considered a pKb of 4.75 for ammonia and a pKb of 10.25 for acetic acid.

XeLa
Title: Re: Weak acid and weak base mixture
Post by: Borek on November 03, 2017, 08:38:32 AM
Quote from: XeLa. on November 03, 2017, 07:39:11 AM
I considered a pKb of 4.75 for ammonia and a pKb of 10.25 for acetic acid.

9.13 it is (ionic force ignored, exact equilibrium).
Title: Re: Weak acid and weak base mixture
Post by: XeLa. on November 03, 2017, 10:12:32 AM
Okay, awesome! Maybe I should use a different pH calculator in future then? Which one did you use, Borek?
Title: Re: Weak acid and weak base mixture
Post by: Borek on November 03, 2017, 11:50:05 AM
http://www.chembuddy.com/?left=BATE&right=pH-calculator

but this program is more general and much more powerful:

http://www.chembuddy.com/?left=Buffer-Maker&right=buffer-calculator