JEE Advanced 2020 — Paper 2 — Question 28

$$\underset{(\text { weak base })}{\mathrm{B}}+\underset{\text { (strong acid) })}{\mathrm{HAA}} \longrightarrow \mathrm{BH}^{+}+\mathrm{A}^{-}$$

From the graph, it is clear, that the equivalence point is reached at a titre value of 6 mL i.e. when 6 mL of HA are

added base (B) is completely neutralized. So it will be half neutralized at titre value of 3 mL ; when 3 mL of HA is

added, ' $B$ ' is half - neutralized. So at this stage it will form a best buffer. i.e. $\mathrm{pOH}=\mathrm{pK} \mathrm{b}_{\mathrm{b}}=3$

So, $\mathrm{pK}_{\mathrm{b}}$ of weak base $=3$