The Ka for HOBr is 2.4 x 10-9. Thus, you can report the pKb for OBr– as

a) 4.2 x 10-6
b) 9.62
c) 5.38
d) 8.62
e) 1.38

help!

Have you forgotten KaKb = Kw?

Convert between Ka and Kb easily.