Final answer:
To calculate the pH at the equivalence point of a titration between a weak acid and a strong base or a weak base and a strong acid, use the stoichiometry of neutralization and the Henderson-Hasselbalch equation, considering the hydrolysis of the conjugate base or acid formed.
Step-by-step explanation:
To calculate the pH at the equivalence point for a titration between a weak acid and a strong base, such as benzoic acid and sodium hydroxide (NaOH), we need to know the amount of acid that has been neutralized and the pKa of the weak acid. For benzoic acid, consider the moles of acid and the stoichiometry of the reaction when it's titrated with the strong base NaOH. Upon reaching the equivalence point, we have a solution of the conjugate base of the acid. The pH can then be determined using the Henderson-Hasselbalch equation.
Similarly, for the titration involving ammonia (NH3) and HCl, we consider the moles of weak base neutralized by the strong acid to form the conjugate acid, ammonium chloride (NH4Cl). Again, the Henderson-Hasselbalch equation is used to calculate the pH at the equivalence point.
It's critical to understand that the equivalence point pH for a weak acid-strong base titration usually is greater than 7 due to the resulting conjugate base's hydrolysis in water, whereas the equivalence point pH for a strong acid-weak base titration typically is less than 7 due to the resulting conjugate acid's hydrolysis.
Exercise problems that require calculating pH during various titration stages can help illustrate these concepts and provide practice with calculations using the corresponding equations and concepts, such as buffer capacity and the pKa or pKb values.