1. Understand the Buffer System

* Lactic acid (HC3H5O3) is a weak acid.

* Sodium lactate (NaC3H5O3) is the salt of its conjugate base, lactate ion (C3H5O3-).

2. Use the Henderson-Hasselbalch Equation

The pH of a buffer solution is determined by the Henderson-Hasselbalch equation:

pH = pKa + log ([conjugate base]/[weak acid])

* pKa: The negative logarithm of the acid dissociation constant (Ka) for lactic acid. You can find this value in a table or online. The pKa of lactic acid is approximately 3.86.

* [conjugate base]: The concentration of lactate ions (C3H5O3-).

* [weak acid]: The concentration of lactic acid (HC3H5O3).

3. Calculate the Concentrations of the Acid and Conjugate Base

* Lactic acid:

* Moles of lactic acid = (0.085 L) * (0.13 mol/L) = 0.01105 mol

* Concentration of lactic acid in the buffer = (0.01105 mol) / (0.085 L + 0.095 L) = 0.0614 mol/L

* Lactate ion:

* Moles of sodium lactate = (0.095 L) * (0.15 mol/L) = 0.01425 mol

* Concentration of lactate ion in the buffer = (0.01425 mol) / (0.085 L + 0.095 L) = 0.0792 mol/L

4. Plug the Values into the Equation

pH = 3.86 + log (0.0792 / 0.0614)

pH ≈ 3.86 + 0.11

pH ≈ 3.97

Therefore, the pH of the buffer solution is approximately 3.97.