StellarpH and pKa
The relative amounts of the acid form and the base form in solution depend on the relationship between the solution pH and the acid’s pKa. When pH < pKa, the protonated acid form is present in higher concentration. When pH > pKa, the deprotonated base form is present in higher concentration. This makes pKa a very useful reference point for predicting which form of an acid-base pair dominates at a given pH. This can be more intuitive mathematically in Lesson 8.9.
This topic also connects to acid-base indicators. Indicators are substances whose protonated and deprotonated forms have different properties, often different colors. To choose a good indicator for a titration, you want an indicator whose pKa is close to the pH near the equivalence point. That ensures the indicator changes appearance in the most useful part of the titration.