Calculating the pH of a Buffered Solution
To study the effect of a weakly acidic culture medium on the growth of a certain strain of bacterium, a microbiologist prepared a buffer solution by making it with 0.11 mol/L NaCH3COO, sodium acetate, and also 0.09 mol/L CH3COOH, acetic acid, What is the final pH?
 
Ka for acetic acid = 1.8 x 10-5
 
Therefore pKa = 4.74
 
[acid] = 0.090 mol/L      [anion] = 0.11 mol/L

pH = 4.74 + log (0.11) = 4.74 + log 1.2 = 4.74 + 0.079 = 4.82
                          (0.09)
 

You may sometimes see the Henderson-Hasselbach equation given in some chemistry sources as pH = pKa + log [salt]/[acid]. DON'T USE IT! It only works for monovalent salts. Use the formula given previously in these notes.