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 HSO3- is 12.19.

My confusion arises from why the pKb for HSO3- is not 7.09. I thought...

pKw=pKa+pKb

14=6.91+pKb

7.09=pKb of HSO3-

What is the above pKb value if not the pKb of HSO3-? I don't understand, conceptually, how it makes sense that you would find the pKb of HSO3- by plugging in the pKa of H2SO3 as required...

pKw=pKa+pKb

14=1.81+pKb

12.19=pKb of HSO3-

Could someone kindly give me an explanation of how this makes sense? I'm missing the intuition behind this!

Thank you very much in advance!

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 HSO3- is 12.19.

My confusion arises from why the pKb for HSO3- is not 7.09. I thought...

pKw=pKa+pKb

14=6.91+pKb

7.09=pKb of HSO3-

What is the above pKb value if not the pKb of HSO3-? I don't understand, conceptually, how it makes sense that you would find the pKb of HSO3- by plugging in the pKa of H2SO3 as required...

pKw=pKa+pKb

14=1.81+pKb

12.19=pKb of HSO3-

Could someone kindly give me an explanation of how this makes sense? I'm missing the intuition behind this!

Thank you very much in advance!