Final answer:
Observing multiple equivalence points during the titration of a weak base with HCl indicates the presence of a polyprotic base that can accept more than one proton. As the weak base is titrated, various species reach points of neutralization at different stages, resulting in multiple equivalence points.
Step-by-step explanation:
When you're plotting the titration data of a weak base with HCl (hydrochloric acid) and observe multiple equivalence points, with the last one occurring at around pH 3, it suggests that you are dealing with a polyprotic base that can accept more than one proton. In a typical strong acid-weak base titration, the pH at the equivalence point is less than 7, which aligns with your observation of a pH around 3. The variation in equivalence points signifies the presence of multiple species in the weak base that can be protonated. The identity of the weak base being titrated significantly affects the shape of the titration curve, with each equivalence point corresponding to the neutralization of one acidic hydrogen.
Consider the titration curve for NH3, a weak base, which is an inverse of the titration curve for a weak acid like acetic acid. Here, a buffer solution forms around the midpoint of the titration curve. As a weak base accepts protons from the strong acid, different species in the weak base solution reach points of neutralization at different stages, hence the multiple equivalence points in your titration curve.