250 mL of 1.0M lactic acid (C₃H₆O₃) is mixed with 500 mL of 1.0M lactate ion (C₃H₅O₃⁻). The Ka for lactic acid is 1.38 x 10⁻⁴. HCl is added to the solution, yet the pH changes very little. Briefly explain how this is accomplished.