A 0·02M solution of pyridinium hydrochloride has pH = 3·44. Calculate the ionisation constant of pyridine.

Solution

Pyridine hydrochloride is a salt of a weak base and strong acid. Hence

space space space space space space pH space equals space minus 1 half open square brackets log space straight K subscript straight w space minus space log space straight K subscript straight b space plus space log space straight C close square brackets

or space space 3.44 space equals space minus 1 half open square brackets negative 14 minus log space straight K subscript straight b space plus space log space left parenthesis 2 space cross times space 10 to the power of negative 2 end exponent right parenthesis close square brackets or space space space 6.88 space equals space 14 space plus space log space straight K subscript straight b space plus space 1.70 or space space log space straight K subscript straight b space equals space minus 8.82 space equals space minus 8 minus 1 plus 1 space minus space 0.82 space equals space 9 with bar on top.18 space space space space space space straight K subscript straight b space equals space antilog space left parenthesis 9 with bar on top space.18 right parenthesis space equals space 1.5 space cross times space 10 to the power of negative 9 end exponent

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