During the titration of 50.0 mL of 0.100 M monoprotic weak acid HA with 0.100 M NaOH, the pH measured at the equivalence point is 9.10. Which of the following is closest to the acid’s dissociation constant, Ka ?
a) 1.3×10−5
b) 8×10−10
c) 3×10−6
d) 6×10−6