@adianadiadi is right - let's say we have an acid (HA) which dissociates into its conjugate base (A-). The dissociation reaction is:

HA --> A- + H+

Let's say there is

**0% dissociation**. That gives us:

HA --> A- + H+

100% --> 0%

Therefore [base]/[acid] or [A-]/[HA] is

**0/100**
Now let's say there is

**10% dissociation**. That gives us:

HA --> A- + H+

90% --> 10%

Therefore [base]/[acid] or [A-]/[HA] is

**10/90**
Now let's say there is

**50% dissociatio**n. That gives us:

HA --> A- + H+

50% --> 50%

Therefore [base]/[acid] or [A-]/[HA] is

**50/50 **
It turns out that at

**50% dissociation**, we have a really intersting situation. The Henderson-Hasselbalch equation states:

pH = pKa + log ([base]/[acid])

What's really cool is that when half is in the acid form, and the other half is in the conjugate base form, the base:acid ratio cancels out!

pH = pKa + log(50/50)

pH = pKa + log(1)

pH = pKa + 0

pH = pKa

The significance of this is the half-equivalence point: basically at this point on a titration curve (the flat parts of the curve), the acid is 50% dissociated. And, if we measure the pH, we can figure out experimentally what its pKa value is! So this is really important because it helps us measure how strong a given acid is, essentially. In other words,

**pH = pKa at the half-equivalence point**. But I digress...

Finally, let's say there is

**100% dissociation**. That gives us:

HA --> A- + H+

0% --> 100%

Therefore [base]/[acid] or [A-]/[HA] is

**100/0** (woops, that's not even a real number - that's probably why the question asks you to stop at 99%

).