400 Ml of an Acetic Acid of Unknown Concentration Is

Multiple Choice

40.0 ml of an acetic acid of unknown concentration is titrated with 0.100 M NaOH. After 20.0 mL of the base solution has been added, the pH in the titration flask is 5.10. What was the concentration of the original acetic acid solution? [K_a(CH₃COOH) = 1.8 * 10^-5]

A) 0.11 M
B) 0.022 M
C) 0.072 M
D) 0.050 M
E) 0.015 M