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
--
Chemical calculators at www.chembuddy.com
pH calculation
concentration conversion

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...