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

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 is true?

A) At pH 4.7,half 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 is correct.