Question

A solution of $0.1M$ weak base $(B)$ is titrated with $0.1M$ of a strong acid $(HA)$. The variation of $pH$ of the solution with the volume of $HA$ added is shown in the figure below. What is the $p{K}_b$ of the base? The neutralization reaction is given by $B+HA\to B{H}^{+}+A^{-}$.

Answer 1

Answer: 03.00

$\underset{\text{ (weak base) }}{B}+\underset{\text{ (strong acid) }}{\mathrm{HA}}⟶B{H}^{+}+A^{-}$
From the graph, it is clear, that the equivalence point is reached at a titre value of $6mL$
i.e. when $6mL$ of HA are added base (B) is completely neutralized.
So it will be half neutralized at titre value of $3mL$;
when $3mL$ of HA is added, 'B' is half-neutralized.
So at this stage it will form a best buffer.
i.e. $pOH=p{K}_b=3$
So, $p{K}_b$ of weak base $=3$