Calculate the percent ionization of 0.125 M lactic acid in a solution containing 8.5×10−3 M sodium lactate.

Question

Express your answer using two significant figures.

Answer 1

0.125 M lactic acid + 8.5x10^-3 M sodium lactate forms a buffer solution. The Ka for lactic acid (HC₃H₅O₃) is 8.3x10^-4, thus pKa = 3.08.

Henderson-Hasselbalch states pH = pKa + log [salt]/[acid]

pH = 3.08 + log (8.5x10^-3)/(0.125)

pH = 3.08 + log 0.068 = 3.08 + (-1.17)

pH = 1.91

[H⁺] = 1x10^-1.91 = 1.23x10^-2 M = 0.0123 M

% ionization = 0.0123 M/0.125 M (x100%) = 9.8%