How to calculate pH of buffer solutions?

Question

Calculate the pH of the following two buffer solutions:

 

(a) 2.0 M CH₃COONa/1.8 M CH₃COOH.

 

(b) 0.2 M CH₃COONa/0.2 M CH₃COOH.

 

Answer 1

The Henderson-Hasselbalch equation is useful here. The problem does not state volumes, only molarity. So equal volumes will be assumed.

a) pH = pKa + log ( [ Ac- ] / [H Ac] ) = 4.74 + log ( 2 / 1.8 ) = 4.79

b) pH = pKa + log( [Ac-] / [H Ac] ) = 4.74 + log (.2 /.2) = 4.74

Here Ac is CH3COO and I have used the handbook value that the pKa for acetic acid is 4.74