Page 1 of 1

Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 5:40 pm
by Ariana de Souza 4C
I'm confused about why when a problem gives you Ka, you know it's going to be a weak acid. Why do we know this? Thanks

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 5:49 pm
by Preston_Dang_1B
This is because the Ka is an equilibrium constant for the dissociation of the acid in water. If you're given any value of Ka, then that means not all of the acid has dissociated when the acid placed in water and there are still molecules of the acid inside of the water, along with some small concentrations of H+ ions and the acid's conjugate base. This is one of the properties of a weak acid. If instead a strong acid was used, it would dissociate completely and there would only be concentrations of H+ ions and the conjugate base in the water and there would be no Ka value because we are able to figure out the exact concentration of H+ and conjugate base ions since there is full dissociation.

Hope this clears things up!

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 5:55 pm
by Tara_Hekmati_3B
We know this because if you had a strong acid, then the acid would dissociate completely in the reaction and there would be no more reactant left. Therefore, the dissociation constant (products/reactants) would have a zero in the denominator and is undefined. This shows why strong acids don't have a Ka. However, if the acid is weak, the reactant only dissociates partially, and allows for a Ka to be calculated. If the acid has a Ka, you know it's a weak acid because that's the only type of acid that can have a calculated Ka.

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 6:24 pm
by Ariana de Souza 4C
but how come, then, when we are given a table of Ka values, we can choose the strongest acid based on the biggest Ka value? Why is there a Ka value then for a strong acid?

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 6:31 pm
by Rachel_Smith_3D
One acid can be stronger than another acid and still not be a strong acid (one that dissociates completely in water). There are varying levels of acidity among weak acids.

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 6:41 pm
by Kristina Mercolino 3C
Why when given Kb is it a weak base?

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 6:55 pm
by Ariana de Souza 4C
okay, thanks guys!

Re: Why when given Ka = weak acid

Posted: Mon Nov 21, 2016 10:42 pm
by Myra_Zhan_2N
If you're given Kb, it's the same case as Ka. There will only be an equilibrium constant if the base is not completely ionized in water. Therefore, Kb must be a weak base.