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The ionisation of a weak acid is
The ionisation of a weak acid is
−
AH<sup>+</sup> =<sup>'''Ka'''</sup> A<sup>-</sup> + H<sup>-</sup>
+
AH<sup>+</sup> =<sup>'''K<sub>a</sub>'''</sup> A<sup>-</sup> + H<sup>-</sup>
+
+
and its dissociation constant '''pK<sub>a</sub> is given by the Henderson-Hasselbach equation:
+
+
pK<sub>a</sub> = pH + log<sub>10</sub> ([AH]/[A<sup>-</sup>)