A buffer solution is prepared by mixing 23.8 mL of 0.0410 M pyridinium chloride
ID: 532316 • Letter: A
Question
A buffer solution is prepared by mixing 23.8 mL of 0.0410 M pyridinium chloride with 62.7 mL of 0.646 M pyridine.
A table of pKa values can be found here.
1. Calculate the pH (to two decimal places) of this solution.
Assume the 5% approximation is valid and that the volumes are additive.
2. Calculate the pH (to two decimal places) of the buffer solution after the addition of 287 mL of a 0.108 M solution of perchloric acid to the existing buffer solution.
Assume the 5% approximation is valid and that the volumes are additive.
Explanation / Answer
1. For the buffer solution,
pH = 5.25 + log(0.646 M x 62.7 ml/0.0410 M x 23.8 ml)
= 6.87
2. when perchloric acid = 0.108 M x 287 ml = 31 mmol is added
pH = 5.25 + log[(0.646 M x 62.7 ml - 31)/(0.0410 M x 23.8 ml + 31)]
= 4.72
pH in 1. is calculated using Hendersen-Hasselbalck equation.
When acid is added, the conjugate base reacts with it and concentration of conjugate base decreases in solution, whereas, the concentration of acid increases in the buffer solution.