Calculate the pH of a buffer solution prepared by mixing 75.0 mL of 1.25 M lactic acid and 40.0 mL of 0.75 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.)

pH = 3.37

Explanation:

A weak acid as lactic acid in a solution with sodium lactate (Its conjugate base) produce a buffer. The pH of this buffer is obtained using H-H equation:

pH = pKa + log [A-] / [HA]

Where pH is the pH of the buffer,

pKa is 3.86

And [] could be taken as the moles of A- = Sodium lactate and HA = Lactic acid.

Moles A-:

40.0mL = 0.0400L * (0.75mol / L) = 0.030 moles A-

Moles HA:

75.0mL = 0.0750L * (1.25mol / L) = 0.09375 moles HA

Replacing:

pH = 3.86+ log [0.030 moles A-] / [0.09375 moles HA]