[3mpathogens]

I have to find the pH of a certain formal concentration of triethylammonium bromide, but the appendix of my book only gives me the pK_a and K_a of triethylamine. How do I find pK_a for the salt?

[Borek]

You do know the Bronsted-Lowry theory of acids and bases?

[3mpathogens]

You do know the Bronsted-Lowry theory of acids and bases?

Well, at least as far as how it defines acids and bases. Is there something I'm missing?

[Borek]

There is no such thing as Ka/pKa of a "salt" that is dissociated in the solution. Ka is a property of a molecule or an ion. What does triethylammonium bromide dissociate into? Is one of the produced ions an acid or a base?

[3mpathogens]

There is no such thing as Ka/pKa of a "salt" that is dissociated in the solution. Ka is a property of a molecule or an ion. What does triethylammonium bromide dissociate into? Is one of the produced ions an acid or a base?

It would dissociate into a triethylamine cation and chloride anion. So Ka is a property of triethylamine. But wouldn't salts of triethylamine dissociate differently depending on the ion paired with triethylamine? For instance, would I use the same Ka (that of triethylamine) for triethylammonium chloride and triethylammonium bromide? Or would they each have a different associated Ka? (And if so, how do you find that Ka?)

Maybe I don't have a clear understanding of how all of this works in general... what should I review to get a better grasp on this? You mentioned the Bronsted-Lowry theory, but is there anything else that would be helpful?

[Borek]

It would dissociate into a triethylamine cation and chloride anion.

Bromide, but it is just a simple, obvious mistake.

wouldn't salts of triethylamine dissociate differently depending on the ion paired with triethylamine?

No (there are some fine prints, but they can be safely ignored here).

For instance, would I use the same Ka (that of triethylamine) for triethylammonium chloride and triethylammonium bromide?

Yes.

It may happen that the counterion itself is a weak/acid base and its reactions have to be taken into account as well (think ammonium acetate, or something like NH₄HCO₃, where HCO₃^- is both an acid and base at the same time). It doesn't change the fact that each ion has its own Ka/Kb which doesn't depend on the identity of other ions - quite the opposite, it uses exactly this approach.

Maybe I don't have a clear understanding of how all of this works in general... what should I review to get a better grasp on this? You mentioned the Bronsted-Lowry theory, but is there anything else that would be helpful?

GenChem 101 - equilibrium and acid-base equilibrium.