Question about acetic acid buffer solutions

[mesa]

I want to make sure I understand the relationship between OH- and COOH properly.

So if the NaOH is introduced into a buffer solution of acetic acid and it dissociates does the OH- (due to it's very electronegative nature) immediately rip off the H+ from the COOH? If I want to calculate pH do I just subtract the mols of NaOH from mols CH3COOH then calculate the pH from the remaining acetic acid (in molarity) and it's corresponding Ka (1.75X10^(-5))?

 

[Borek]

Yes, when dealing with buffers you can assume neutralization went to completion (and was stoichiometric). In some specific cases that's not exactly true, but as long as the pH calculated is less than 1 (or even 2) pH units from pKa, you are usually on the safe side.

 

[mesa]

Yes, when dealing with buffers you can assume neutralization went to completion (and was stoichiometric). In some specific cases that's not exactly true, but as long as the pH calculated is less than 1 (or even 2) pH units from pKa, you are usually on the safe side.

Good deal, interesting stuff to think about.
So the cutoff for being able to do this is not clearly defined. It would be fun to play with one of the schools pH meters and explore this further. For what I am doing the difference is 1.82 pH units from pKa, within range for NaOH correct?

So for a weak base do we calculate mols OH- produced using it's corresponding Kb and subtract that from the calculated H+ mols then take the -log of the new molarity of H30+?

 

[Borek]

For not to concentrated buffers it is not difficult to test these things just by calculating pH of the solution, although I would not use Henderson-Hasselbalch equation but a full approach, compare http://www.chembuddy.com/?left=pH-calculation&right=toc. Alternatively, you can use one of my pH calculators for calculations - either BATE or the one built in Buffer Maker (there are free 30-days trials).

1.82 pH unit from pKa for acetic acid - can work, although if you need it to buffer the solution I would look for another buffer. The closer to the pKa you are the more effective the buffer is (the higher is its buffering capacity).

 

[mesa]

That is a good website, interesting how it is not actually the molarity but the activity of the hydronium that is really changing the pH. No surprise that there is still a fairly strong correlation of course.

I like doing this kind of stuff pen to paper but your online calculators would be good to check if I am getting it right until I get more comfortable.

Very helpful.