A 0.02 M solution of pyridinium hydrochloride has pH=3.44. calculate the ionisation constant of pyridine.

Explanation

Pyridinium hydrochloride is a salt of weak base and strong acid. For the solution of such a salt,
`pH=7-(1)/(2)pK_(b)-(1)/(2)logc`
Given: `pH=3.44 and c=0.02M`
`therefore 3.44=7-(1)/(2)pK_(b)-(1)/(2)log0.02`
or, `3.44=7-(1)/(2)pK_(b)+0.85 or, pK_(b)=8.82`
`therefore K_(b)=10^(-pK_(b))=10^(-8.82)=1.51xx10^(-9)`.