A buffer solution contains 0.100 mole of propionic acid and 0.130 mole of cesium propionate (CsC3H5O2) in 2.00 L of solution.

Question

a. What is the pH of this buffer?

b. What is the pH of this buffer after the addition of 0.015 mole of HI?

Answer 1

Ka propionic acid = 1.34x10^-5 (looked up in a table)

pKa = 4.89

[HC₃H₅O₂] = 0.100 mol / 2.00 L = 0.0500 M

[CsC₃H₅O₂] = 0.130 mol / 2.00 L = 0.065 M

Henderson Hasselbalch equation: pH = pKa + log [salt] / [acid]

pH = 4.89 + log (0.065/0.0500)

pH = 4.89 + log 1.3 = 4.89 + 0.11

pH = 5.00

After addition of 0.015 mol HI...

[HC₃H₅O₂] = 0.0500 + 0.015 = 0.065 M

[CsC₃H₅O₂] = 0.065 - 0.015 = 0.050 M

pH = 489 + log (0.050 / 0.065)

pH = 4.89 + log 0.769 = 4.89 -0.11

pH = 4.78