pkb

Hi guys, This may be a silly question, but I'm confused about the formula pKw=pKa+pKb (and I guess by extension Kw=Ka*Kb). My confusion came from an Examkrackers question asking "What is the pKb for HSO3-?" when given the pKa of H2SO3 (1.81) and pKa of HSO3- (6.91). The answer for the pKb of...