# Conjugate acids and bases

1. Jun 8, 2005

### 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.

2. Jun 9, 2005

### Staff: Mentor

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.

3. Jun 9, 2005

### 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?

4. Jun 9, 2005

### quetzalcoatl9

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

5. Jun 9, 2005

### Staff: Mentor

Write down any pair of acid and conjugate base.

6. Jun 10, 2005

### Xishan

Why is it so? can plz 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?

7. Jun 10, 2005

### Staff: Mentor

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:

$$Ka Kb = \frac {[H^+][A^-]}{[HA]} \frac {[HA][OH^-]}{[A^-]} = [H^+][OH^-] = Kw$$

In other words (or symbols):

$$pKa + pKb = pKw$$

The stronger the acid, the weaker the conjugated base.

8. Jun 11, 2005

### 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?