[CHM 112] CHM 112 Buffer Calculation Question

[Andrew J. Pounds]

On 4/14/26 13:30, wrote:
> Dr. Pounds,
> I am studying for our exam tomorrow and I have run into a quick 
> question. When looking at the problems we worked in class during 
> chapter 17, I am confused about how we calculated some of the initial 
> concentrations. For example, on slide 13 of the Chapter 17 lecture, 
> how do I know what to add and subtract from the initial moles of 
> acetate and acidic acids to get their new initial concentrations? I 
> know that it says that you calculate the one that is decreasing first, 
> but how do I know which is decreasing? Also, what makes this problem 
> different so that I know that these types of calculations are needed? 
> Earlier in that lecture, on slide 9, we added HCl to a solution but 
> didn't have to add or subtract anything extra during the 
> concentrations. I am assuming that the sodium acetate in the problem 
> on 13 is the reason for that, but why exactly does that change it?
> Sorry for the long email, I am hoping it is easier this way but if you 
> would rather us meet in person, I am free for most of the afternoon.
> Thank you so much for the help!
> Respectfully,


Great questions! Let's start with slide 13.  Look at the equilibrium 
chemical equation.  Remember, before we start to add anything that 
system has reached equilibrium.  If we add a strong acid we are 
essentialy adding H3O+ to the system so Le Chatelier's principle says 
that the system must shift back toward reactants to regain equilibrium.  
If I shift towards reactants then the acetate ion will be decreasing and 
the acetic acid will be increasing.


Now this is the interesting part... in slide 9 I did not have a common 
ion (like acetate) it is an unbuffered solution.  There was no acetate 
ion to combine with the hydrogen from the HCl to reform acetic acid.  
What we were demonstrating there was how much the pH would drop just by 
adding a small amount of HCl.


Notice though that the ICE table is always written so that the acetic 
acid is dissociating for the H3O+ and the acetate ion -- because that is 
what the Ka value represents.


Let me know if you need further clarification.



> -- 
*/Andrew J. Pounds, Ph.D./*
/Professor of Chemistry and Computer Science/
/Director of the Computational Science Program/
/Mercer University/
/1501 Mercer University Drive, Macon, GA 31207 /
/(478) 301-5627/
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