Question

How does pH relate to pKa in a titration?

Answer 1

The pH at half-equivalence when a weak acid is titrated with a strong base is precisely the pKa of the acid.

Explanation:

Consider the dissociation of a weak acid, HA:

HA⇌H++A−. As we know,

Ka=[H+][A−][HA],

andlog10{Ka}=log10[H+]+log10{[A−][HA]};

Equivalently (multiplying each by −1,

−log10{Ka}=−log10[H+]−log10{[A−][HA]}.

But, by definition, −log10{Ka}=pKa, and −log10[H+]=pH.

Therefore, pH = pKa + log

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