Textbook Problem J.17

[Angelica Soriano 3L]

Hi, I'd just like some clarification on this problem:

In each of the following salts, either the cation or the anion is a weak acid or a weak base in water. Write the chemical equation for the proton transfer reaction of this cation or anion with water: (a) NaC6H5O

I went about this problem knowing that Na+ won't affect the pH of the solution, so I focused on C6H5O-. I knew that oxygen has a partial negative charge on it so the reaction with water would look like this: C6H5O- + H2O --> C6H5OH + OH-

In the question it said that either the cation or anion will be a weak base. I was just a bit confused because I assumed that weak acids (in this problem, C6H5OH) had relatively strong conjugate bases (C6H5O-). Since C6H5O- is gaining a proton, would it be considered a weak base?

[Hazelle Gunawan 3G]

My assumption before reading this post was that weak acids generally tended to correspond with weak conjugate bases, so it's interesting that you said the opposite! Would be interested in seeing what anyone else has to say on this.

[Katherine_Douglas_1F]

I'm not exactly sure what the correlation between weak acids and their conjugate bases is, but in this problem, C6H5O- is a weak base. The question is pretty much just asking whether the salt is going to make the pH of the water more acidic or more basic. Since C6H5O- attracts H+ and leaves OH-, it can be concluded that the salt will make the pH of the water increase. This means that, in water, C6H5O- is acting as a weak base as it accepts H+ and raises the overall pH.

[Nicole Bruno Dis 1B]

The anion of NaC6H5O is basic in nature. Therefore, the chemical equation for its proton transfer is: C6H50- + H2O ---> C6H5OH + OH-. Hence, NaC6H5O is a salt of strong base and weak acid.