Since all of these differ by more than 1 H+, they do NOT qualify as conjugate
pairs.
Now, the member of a conjugate pair that has an extra H+ is called the conjugate
acid, and the member that has one fewer H+ is the conjugate base.
A word of caution: Just because a molecule is called conjugate acid or base
does not mean it’s actually acidic or basic in solution.
For example, given the conjugate pair
OH−/O2−we see that by definition, hydroxide is the conjugate acid of the pair, but of
course, OH− is actually a base in solution.
Calculating Equilibrium Constants (K) for Weak Conjugate
Acid/Base Pairs
Keep in mind that weak acids and bases do not completely ionize in aqueous
solution. There is a measurable equilibrium, called the ionic equilibrium,
between the weak acid and its conjugate base (or between the weak base and its
conjugate acid). Here’s a derivation that is pretty important to look through
before test day. If you take any conjugate pair of a weak acid and weak base,
such as NH 4 + and NH 3 , writing out the balanced dissociation reactions in water
for each gives the following:
For NH 4 +(aq): NH 4 +(aq) H+(aq) + NH 3 (aq)For NH 3 (aq): NH 3 (aq) + H 2 O(l) NH 4 +(aq) + OH−(aq)
Then, writing out the equilibrium expressions for each gives
For NH 4 +(aq): pKaNH 4 + = [NH 3 ][H+]/[NH 4 +]