March 29, 2024, 07:48:57 AM
Forum Rules: Read This Before Posting


Topic: How do you find the equilibrium constant of a salt?  (Read 2402 times)

0 Members and 1 Guest are viewing this topic.

Offline 3mpathogens

  • New Member
  • **
  • Posts: 6
  • Mole Snacks: +0/-0
How do you find the equilibrium constant of a salt?
« 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 pKa and Ka of triethylamine. How do I find pKa for the salt?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: How do you find the equilibrium constant of salt?
« Reply #1 on: August 21, 2021, 06:08:57 PM »
You do know the Bronsted-Lowry theory of acids and bases?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline 3mpathogens

  • New Member
  • **
  • Posts: 6
  • Mole Snacks: +0/-0
Re: How do you find the equilibrium constant of salt?
« Reply #2 on: August 21, 2021, 06:19:50 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?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: How do you find the equilibrium constant of a salt?
« Reply #3 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?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline 3mpathogens

  • New Member
  • **
  • Posts: 6
  • Mole Snacks: +0/-0
Re: How do you find the equilibrium constant of a salt?
« Reply #4 on: August 22, 2021, 10:18:48 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?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: How do you find the equilibrium constant of a salt?
« Reply #5 on: August 22, 2021, 11:04:32 AM »
It would dissociate into a triethylamine cation and chloride anion.

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

Quote
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).

Quote
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.

Quote
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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links