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. What is Ka2 for ascorbic acid?
mL NaOH added 10.00 20.00 30.00 40.00
pH 4.17 5.21 11.55 12.89
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
The volumes listed correspond to the ½ equivalence point, equivalence point, 1 ½ equivalence point, and 2 equivalence point. The pH will equal pKa2 at the 1 ½ equivalence point, which is 11.55. Therefore, the [H+] = 10^(-11.55) = 2.81 x 10-12