Question

Define the equivalence point of an acid-base titration. Describe it for a weak acid-strong base titration (will the pH be >,<, or = 7).

Answer 1

Answer: As HCl is added, the H+ from the strong acid reacts with the best base present, NH3. Because something strong is reacted, we assume the reaction goes to completion. The reaction used for the stoichiometry part of the problem is: H+ + NH3 ? NH4 + . Note that the effect of this reaction is to convert the weak base into its conjugate acid. The various parts to a weak basestrong acid titration are: a. Initially before any strong acid is added. We have a weak base in water. Write out the Kb reaction for the weak base, set-up the ICE table, and then solve.

b. From 0.1 mL HCl added to just gefore the equivalence point (49.9 mL HCl added). The major species present in this region are NH3, NH4 + , Cl? , and water. We have a weak base and its conjugate acid present at the same time; we have a buffer. Solve the buffer problem using the Ka reaction for NH4 + , the Kb reaction for NH3, or the HendersonHasselbalch equation. The special point in the buffer region of the titration is the halfway point to equivalence. Here, [NH3] = [NH4 + ], so pH = pKa = !log (5.6