March 4, 2015

Homework Help: Stupid Chemistry

Posted by Taasha on Saturday, August 4, 2007 at 10:17pm.

Ok, blonde moment here, Kb is the same as Ka but just for bases and Ka is for acids correct?

right. Ka is the ionization constant for acids, Kb is the ionization constant for bases.
BUT, we can change one into the other this way.
KaKb = Kw.
So if you are given a Ka but you need the Kb for the same acid, KaKb = Kw will convert it for you.

Okay, now that is settled. Thanks. If I'm trying to figure out the pH of a solution and am given the formula for the species, the conjugate acid, and the Kb, do I use the original formula or the conjugate acid formula when I convert to mol/L?

It confusing to answer a question like that. Suppose we want the pH of a solution that is 0.1 M in acetate ion, which I will label Ac^- but you realize it is C2H3O2^- and the acid is HC2H3O2.
We write the hydrolysis equation.
Ac^- + HOH ==> HAc + OH^-
Then Kb (the Ac^- acts as a base) =
[(HAc)(OH^-)/(Ac^-)] = Kb.
Usually we are given the Ka for acetic acid and we do Kb = Kw/Ka and plug in Kb. Ka is the ionization constant for acetic acid, Kb is the base constant for the acetate ion, the conjucate base.

Okay. Thanks. Now, how do I calculate the percent ionization of a reaction. For example, if I have a 0.21 M acid with a Ka of 1.3 x 10^-5, what is the percent ionization. I don't even know where to begin. We've never even discussed a problem like that in class.

I think I went through that earlier although it may have been for another student. Anyway, do you have H^+ and P^- ion? If so then [(H^+)/(HP)] x 100 = percent ionization. It's the same as calculating precent apples in a bag of apples and oranges.
%apples = (#apples)/(total fruit)*100.
%ionization = (amount ionized)/(what we started with)*100.

Oh geez, thats easy. It might have been me and maybe I just wasn't listening. By the way, I LOVE how you are explaining to me how to do this rather than just giving me answers. It really helps, as I am "going back to school" and I am trying to do the best I can at really understanding this stuff.

Thanks. We try to explain the concept and not do the actual homework. It's the concept that you need to understand. You can memorize 100 problems and always be able to do those 100 but no others; BUT you can learn the concept and work thousands just like it.

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