Lesson 3: Part 10

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vic2019
Posts: 4
Joined: Fri Aug 16, 2019 12:39 pm

Lesson 3: Part 10

Post by vic2019 » Thu Sep 12, 2019 9:18 pm

Hello!

I have a question regarding #16 in Lesson 3, Part 10, asking us what the pH of a 0.10 M solution of sodium acetate would be. Our instructor said that because acetic acid is a weak acid, acetate should be a weak conjugate base. However, I though the rule was for a weak acid, its conjugate base is strong and for a strong acid, its conjugate base is weak?
I have attached a picture of what was noted for the solution.

Also, should we assume that if an acid that is given in a problem is not one of the 6 notable strong acids, then the acid given in the problem is weak?

Thank you!
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NS_Tutor_Mathias
Posts: 259
Joined: Sat Mar 30, 2019 8:39 pm

Re: Lesson 3: Part 10

Post by NS_Tutor_Mathias » Fri Sep 13, 2019 3:11 am

The pKb of acetate (9.2) tells us it is a weak base.

You are right, there is a common heuristic that is often useful (weak acids produce strong bases etc.) that is good to have in mind when estimating answers. However, like so many things, it is not always true ;) I am not sure the use of "therefore" here is appropriate in the slide.

Speaking of:
Making an actual sodium acetate solution in the lab will give surprising results. Since it lacks buffering capacity for the carbonic acid made from exposure to the atmosphere, the pH of a real-life acetate solution tends to be a good chunk lower than what the equations would predict. Can you guess why this is not the case for solutions of both acetic acid and acetate? (As in, you can predict the pH of such a solution virtually perfectly with just Henderson-Hasselbalch)
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