What Determines the Strength of a Conjugate Acid-Base Pair?

[Dooh]

Uggh finally did all the chem problems and studied for the upcoming final but there are 2 questions left unanswered. MAybe someone can help me out with this:

If given a lists of acids, how would one know which 1 is the weakest conjugate base? For example, HF, HNO_2, H_2CO_3, H_3BO_3, HCl, which 1 would be it? I didn't know to solve for it.

Also, a conjudate acid-base pair consists of two substances that:

-differ by 1 proton
-neutralize each other
-have equal number of protons
-satisfy oth definition
or
-none

BY eliminationg all the choices, i concluded that its none but I'm still not sure.

 

[Borek]

The stronger the acid the weaker the conjugate base. It works both sides - the stronger the base, the weaker the conujgate acid.

For every answer eliminated, explain why, so that we can point you to your error.

 

[Dooh]

I think it's none because none of the options with the definition of an acid-base pair. They obviously don't neutralize each other and the number of protons in this case is irrelevant. That's what i think anyway. Am i right?

 

[quetzalcoatl9]

i dunno, i would imagine that HCl is the strongest acid and therefore Cl- would be the weakest conjugate base.

 

[Borek]

the number of protons in this case is irrelevant

Write down any pair of acid and conjugate base.

 

[Xishan]

The stronger the acid the weaker the conjugate base. It works both sides - the stronger the base, the weaker the conujgate acid.

For every answer eliminated, explain why, so that we can point you to your error.


Borek
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Why is it so? can please give me the reason that why the conjugate base of an strong acid is weak?

When we define a strong acid (or base), we say that it is an acid (or base) which ionizes to a higher extent in acquous solution. What is the reason that it does so? Is the polarity of its bonds that causes the ionization?

 

[Borek]

acid dissociation:

HA \leftrightarrow H^+ + A^-

acid dissociation constant:

Ka = \frac {[H^+][A^-]}{[HA]}

conjugated base dissociation:

A^- + H_2O \leftrightarrow HA + OH^-

conjugated base dissociation constant:

Kb = \frac {[HA][OH^-]}{[A^-]}

water dissociation:

H_2O \leftrightarrow H^+ + OH^-

water dissociation constant:

Kw = [H^+][OH^-]

and now Ka*Kb:

Ka Kb = \frac {[H^+][A^-]}{[HA]} \frac {[HA][OH^-]}{[A^-]} = <br /> [H^+][OH^-] = Kw

In other words (or symbols):

pKa + pKb = pKw

The stronger the acid, the weaker the conjugated base.

 

[Xishan]

Thanks! this answers my first question, now what about the second one: Is it the polarity of bonds that characterizes a strong acid or base or something else? what is that?

Dooh! please forgive me if I'm HACKING your thread...