Final answer:
The Ka of the weak acid can be calculated using the pH provided as the pKa (Ka = 10^-pKa). The exact pH values after 15 mL, 20 mL, and 25 mL of NaOH added cannot be determined without additional information about the weak acid.
Step-by-step explanation:
The question asks for the calculation of the acid dissociation constant (Ka) of a weak acid after titration with a strong base and determining the pH at different volumes of the base added. Since the pH is equal to the pKa at the halfway point to the equivalence point in a titration, we can use the pH of 3.9 provided to determine Ka. However, additional information is needed to determine the pH at volumes of 15 mL, 20 mL, and 25 mL of NaOH added. Without this additional information, such as initial volumes or the identity of the weak acid, further pH calculations cannot be completed.
We were provided with the concept that the pH=the pKa at the halfway point in a titration of a weak acid. Thus, using the pH given after 10 mL of NaOH was added, we conclude that this is the halfway point. So, pKa = pH = 3.9. Once you know the pKa, you can calculate Ka using the formula Ka = 10-pKa.