Calculate the pH of a buffer solution prepared by mixing 50.0 mL of 1.00 M lactic acid and 40.0 mL of 0.50 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 of the buffer is 4,26

Explanation:

The mixture of a weak acid as lactic acid with its conjugate base (Lactate ion), produce a buffer that follows the Henderson-Hasselbalch equation:

pH = pKa + log [A⁻] / [HA]

Where pKa is pKa of lactic acid: 3.86

And [] could be taken as moles of A⁻, sodium lactate and HA lactic acid.

Moles Lactate:

0.050L * (1.00mol /L) = 0.0500moles

Moles lactic acid:

0.040L * (0.50mol / L) = 0.0200 moles

Replacing in H-H equation:

pH = 3.86 + log [0.0500] / [0.0200]

pH = 4.26

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