Calculate the pH of a buffer solution prepared by mixing 60.0 mL of 1.00 M lactic acid and 25.0 mL of 1.00 M sodium lactate.

(The pKa for lactic acid is 3.86. The molar mass for lactic acid is 90.1 g/mol and the molar mass for sodium lactate is 112.1 g/mol.)

This problem could be solved easily using the Henderson-Hasselbach equation used for preparing buffer solutions. The equation is written below:

pH = pKa + log[ (salt/acid]

Where salt represents the molarity of salt (sodium lactate), while acid is the molarity of acid (lactic acid).

Moles of salt = 1 mol/L * 25 mL * 1 L/1000 mL = 0.025 moles salt

Moles of acid = 1 mol/L * 60 mL * 1 L/1000 mL = 0.06 moles acid

Total Volume = (25 mL + 60 mL) * (1 L/1000 mL) = 0.085 L

Molarity of salt = 0.025 mol/0.085 L = 0.29412 M

Molarity of acid = 0.06 mol/0.085 L = 0.70588 M

Thus,

pH = 3.86 + log (0.29412/0.70588)

pH = 3.48