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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: 3mpathogens on August 21, 2021, 05:58:58 PM

Title: How do you find the equilibrium constant of a salt?
Post by: 3mpathogens on August 21, 2021, 05:58:58 PM: 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?
Title: Re: How do you find the equilibrium constant of salt?
Post by: Borek on August 21, 2021, 06:08:57 PM: You do know the Bronsted-Lowry theory of acids and bases?
Title: Re: How do you find the equilibrium constant of salt?
Post by: 3mpathogens on August 21, 2021, 06:19:50 PM: Quote from: Borek on August 21, 2021, 06:08:57 PM
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?
Title: Re: How do you find the equilibrium constant of a salt?
Post by: Borek on August 22, 2021, 03:24:17 AM: 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?
Title: Re: How do you find the equilibrium constant of a salt?
Post by: 3mpathogens on August 22, 2021, 10:18:48 AM: Quote from: Borek on August 22, 2021, 03:24:17 AM
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?
Title: Re: How do you find the equilibrium constant of a salt?
Post by: Borek on August 22, 2021, 11:04:32 AM: Quote from: 3mpathogens on August 22, 2021, 10:18:48 AM
It would dissociate into a triethylamine cation and chloride anion.

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

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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).

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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.

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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.