You carry out the titration of 35.00 mL of 0.1750 M pyridine using a 0.275 M HCl solution. Pyridine has a pKb = 8.77

Question

You carry out the titration of 35.00 mL of 0.1750 M pyridine using a 0.275 M HCl solution. Pyridine has a pKb = 8.77

pH For:
0 mL
5.0 mL
1/2 Equivalence point
17.0 mL
21.0 mL
Equivalence Point
26.0 mL

I worked it all out and my answers were:
0 mL = 9.24
5.0 mL = 10.77
1/2 Equivalence = 5.23
17.0 mL = 2.45
21.0 mL = 5.225
Equivalence = 3.10
26.0 mL = 1.77

I feel like my answers jump around too much and I was wondering if someone could check me or point me in the right direction to get the right pH values. Thanks!

Answer 1

1. You are titrating a weak base against a strong acid, therefore, the pH should decrease continuously during the titration.
2. after the initial stage (0 mL of acid added) till the equivalence point, you have to treat the reaction mixture as a buffer and use Henderson- Hasselbalch equation to calculate the pH.
3. at the equivalence point, all of the base is neutralized, therefore, the reaction mixture is a solution pure weak acid (conjugate acid), and use pKa of the conjugate acid to calculate the pH.
4. after the the equivalence point, the reaction mixture is a mixture of excess strong acid added and the conjugate acid (weak acid), therefore, in most cases you can disregard the H⁺ contribution coming from the weak acid ( strong acids suppress the dissociation of weak acids) and calculate the pH as a solution of a strong acid.