Question

$5mL$ of $0.1mol.{dm}^{-3}$ aqueous pyridine $(K_b=1.7\times {10}^{-9}mol.{dm}^{-3})$ is titrated with $0.1mol.{dm}^{-3}$ hydrochloric acid. Calculate the pH initally, at equivalence and after $30{cm}^3$ hydrochloric acid have been added.

Answer 1

For salt of weak-base and strong acid:

$pH=\frac{1}{2}[p{K}_w-p{K}_b-\log C]$

$p{K}_w=10;$

$p{K}_b=-\log 1.7\times {10}^{-9}=8.769;$

$C=0.05$

at equivalence, $pH=3.27$

$5cc$ $0.1M$ $HCl$ will be considered.

Mole of $HCl$ remaining $=\frac{0.1}{1000}\times 5=5\times {10}^{-4}$

New molarity$=\frac{5\times {10}^{-4}}{55}\times 1000=0.00909M$

$pH=-\log \left[0.00909\right]=2.04$