Carbonic acid, H2CO3, ( pKa1=6.36, pKa2=10.25 ) is titrated with 0.0400 M NaOH. If 25.0 mL of 0.0200 M H2CO3 are used, what is the pH after exactly 18.8 mL of the base have been added (halfway between the first and second equivalence points)?
a) 10.25
b) 6.36
c) 8.25
d) 12.13