PKb of weak base titrated with strong acid

[i_love_science]

Homework Statement:

(see below)

Relevant Equations:

titration curve
pH + pOH = 14

at halfway point:
pH=pKa
pOH=pKb

What is true about the pK of the analyte?
a) pKa = 9.2
b) pKb = 9.2
c) pKb = 4.8
d) pKa = 11.7

The halfway point is at point 4, at approximately pH=9.2. Therefore, the pOH = pKb = 4.8, and I think the answer is c).
The solution says b) is the correct answer. Could anyone explain why, or whether or not my answer is correct? Thanks.

 

[Borek]

Solution is wrong.

Not that you are entirely right, c is not the only correct answer.