Acid and Base PH and Ionization Percent

Question

Calculate the percent ionization of 0.120 M lactic acid in a solution containing 8.5×10⁻³  M sodium lactate.

Answer 1

Let lactic acid be represented by HA and lactate anion by A^-

HA ==> H+ + A-

The solution of HA and A- constitutes a buffer, so we can use the Henderson Hasselbalch equation:

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

Looking up the Ka for lactic acid I find it to be 1.38x10^-4 and so pKa = -log Ka = 3.86

We can now solve for pH:

pH = 3.86 + log (8.5x10^-3/0.12) = 3.86 + (-1.15)

pH = 2.71

From this value we can find the [H⁺]:

[H⁺] = 1x10^-2.71 = 1.95x10^-3 M

Since HA ==> H⁺ + A^- we can now estimate the % ionization as follows:

1.95x10^-3 / 0.120 (x100) = 1.63% ionized