Calculate the pH of a solution that is 0.232 M acetic acid and 0.183 M sodium acetate. The Ka of acetic acid is 1.76×10–5 at 25°C. What is the pH of this mixture at 0°C? (At 0°C, Ka = 1.64x10-5)

Question

A. pH at 25°C = 

B. pH at 0°C =

2 decimal places

Answer 1

Acetic acid is a weak acid

Sodium acetate is the conjugate base of the weak acid.

This solution forms a buffer.

The pH of a buffer can be calculated using the Henderson Hasselbalch equation:

pH = pKa + log [conj.base] / [acid]

pKa = -log Ka = -log 1.76x10^-5

pKa = 4.75

Solving for pH, we have ...

pH = 4.75 + log [0.183/0.232)

pH = 4.75 + log 0.789

pH = 4.75 - 0.103

pH = 4.65 @ 25ºC

To find pH @ 0ºC perform the same calculations using pKa = -log 1.64x10^-5 = 4.79

pH @ 0ºC = 4.69