Halfway point and stoichiometric point

[Chem_Mod]

Question: Why is the significance of the point halfway to the stoichiometric point?

[Chem_Mod]

Answer: The halfway to stoichiometric point is where your pH = pKa. At the stoichiometric point, the number of moles of your initial acid is equal to the number of moles of your strong base. Because of this, the dominate reaction would be: A- + H2O <=> HA + OH-. Using the Kb for this reaction, you can calculate [OH-], which you could then use to calculate pOH and relate it to pH. Another option is to then find [H+] from your [OH-] and then calculate pH. Either way will give you the same answer. But at the stoichiometric point, the pH does NOT equal the pKa.

[Sylvia Li]

The halfway point to the stoichiometric point is also the point at which the weak acid or base acts at its optimum buffer capacity.