[seal308]

I think i understand some things but my textbook doesn't really go into a lot of detail.

I will list what i know and hopefully someone can fill in the blanks. (there are a lot of blanks)

i know that carboxylic acids have the form RCO2H and they are always weak acids
oxoacids have the form HEO where E is some element and they are weak acid
i know that amines are bases.

I've memorized the strong acids and bases.
I'm making an guess, correct me if i'm wrong.
My guess is that for example: compound NH4I
it's major species are NH4+ and I-
i know NH4 + H2O <--> NH3 + H3O is a possible equilibrium
however i am concerned about the other species
my guess is that I- comes from HI (hydroiodic acid) is strong
so that would mean that the reverse (HI --> I) would go to completion
therefore i would expect I to HI would not happen.

The last thing i'm wondering about would be comparing equilbrias of species. Would finding out the Ka, Kb help determine if it's an acid or a base. As in which equilbria is favored.
For example
you have a compound AB where A is an acid and B is a base.
If B has a kb>ka of A then you would know the solution is basic.
Is this right?

again if there are any other ways to determine if it is an acid or a base, it would be much appreciated.
thanks

[iluvlynx]

I'm a bit confused on what your questions are.
First of all, HI is not a strong acid. The only strong acids are the ones you've had to memorize (I believe there are 6). Every other acid is weak.

Well, no, K_a and K_b wouldn't help determine if a species is an acid or base. Actually, it will, but you'll just look at the subscript a and b rather than the number. Because a base wouldn't have a K_a, and vice versa.

But you can see how strong the acids or bases are from the K constants. They're the [products]/[reactants], right? So, look at it from a math perspective. If the K constant is a large number, are there more products than reactants. If the K constant is a tiny number with a large negative exponent, there are more reactants than products at equilibrium.

Also, do you know the three ways to see if a species is an acid or a base? There is the Bronsted-Lowry and the Arrhenius theories (these talk about the H⁺. Acids are the ones that give the other species a hydrogen, also called a proton), and Lewis structure (this one talks about the electrons rather than protons. An acid accepts a pair of electrons from the base.)

[Borek]

First of all, HI is not a strong acid. The only strong acids are the ones you've had to memorize (I believe there are 6). Every other acid is weak.

No, HI is a strong acid.

Well, no, K_a and K_b wouldn't help determine if a species is an acid or base. Actually, it will, but you'll just look at the subscript a and b rather than the number. Because a base wouldn't have a K_a, and vice versa.

No, strength of the base can be given using both Kb and Ka of the conjugate acid, same about acid. pKa + pKb = pKw.

[iluvlynx]

*smacks self* I'm sorry, HI is a strong acid, yes.

No, strength of the base can be given using both Kb and Ka of the conjugate acid, same about acid. pKa + pKb = pKw.

I wasn't referring to the strength, I was just talking about if it's an acid or base. Also (just a note to whomever), to find the pKa or pKb, pK_w can be divided by pK_a or pK_b to find the opposite pK. If that makes sense at all.
(Also, do feel free to correct me, as my memory is a bit rusty.)

[Borek]

I wasn't referring to the strength, I was just talking about if it's an acid or base.

And you were wrong, as it is nothing unusual to see pKa for ammonia, even if ammonia is a base. It is kind of a shortcut, as Ka is given for HN₄⁺, not NH₃, but it still means your approach is wrong.

Also (just a note to whomever), to find the pKa or pKb, pK_w can be divided by pK_a or pK_b to find the opposite pK. If that makes sense at all.

No, it doesn't make sense. You are mixing two things. I already posted the correct formula. It is either

[tex]pK_a + pK_b = pK_w[/tex]

or

[tex]K_a K_b = K_w[/tex]