14B acids and bases section

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Joined: Wed Sep 18, 2019 12:17 am

14B acids and bases section

Postby JosephineF » Tue Mar 10, 2020 1:55 pm

Should we use the 14A section on Acids and Bases for our questions? I am confused about Q3D of the midterm where it asks: at pH 6, what is the net charge of acetic acid, pKa= 4.75? At the review the TA said that you can tell that it will be a -1 charge because the pKa is smaller than the pH. I am confused on the reasoning behind this because wouldn't you need to convert the pKa to pH before you could tell if it was more acidic or more basic when compared to the pH?

Caitlyn Tran 2E
Posts: 100
Joined: Fri Aug 09, 2019 12:15 am

Re: 14B acids and bases section

Postby Caitlyn Tran 2E » Tue Mar 10, 2020 3:17 pm

We did go over this in 14A, but I think you can also think though this using your knowledge of equilibrium. If you get rid of the log for pH and pKa, then you will be left with a concentration of hydronium ions and the Ka. When the pH is lower than pKa, this means that there are more hydronium ions than the hydronium ions in the Ka. To reach the equilibrium represented by Ka, then the acid has to be protonated as HA since there are too many hydronium ions. Similarly, if the pH > pKa, then there are less hydronium ions than in the Ka, so the acid will dissociate to make up for the lower concentration of hydronium ions. Hope this helps!

Daria Azizad 1K
Posts: 116
Joined: Thu Jul 25, 2019 12:15 am

Re: 14B acids and bases section

Postby Daria Azizad 1K » Tue Mar 10, 2020 6:22 pm

Since 14B acids and bases builds upon info from the 14a section, I think it would be useful to review it if necessary.

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