A Student Titrates an Unknown Weak Acid, HA, to a Pale

Multiple Choice

A student titrates an unknown weak acid, HA, to a pale pink phenolphthalein endpoint with 25.0 mL of 0.100 M NaOH. The student then adds 13.0 mL of 0.100 M HCl. The pH of the resulting solution is 4.7. Which of the following statements is true?

A) At pH 4.7, half of the conjugate base, A^-, has been converted to HA.
B) The pK_a of the acid is 4.7.
C) The pK_a of the acid is less than 4.7.
D) The pK_a of the acid is greater than 4.7.
E) More than one of the above statements are correct.