A 200-ML Sample of 0

A 20.0-mL sample of 0.50 M H₂C₆H₆O₆ (ascorbic acid, a diprotic acid) was titrated with 0.50 M NaOH. The following data were gathered during the titration. mL NaOH added 10.00 20.00 30.00 40.00
PH 4.17 5.21 11.55 12.89
What is K_a2 for ascorbic acid?

A) 6.8 × 10^-5
B) 6.2 × 10^-6
C) 6.2 × 10^-7
D) 6.2 × 10^-8
E) 2.8 × 10^-12