Question

Why does pH = pK when pH = 7?

Answer 1

`pH` `=` `pK_a` in a buffer solution WHEN `[HA]=[A^-]`.

Explanation:

When weak acid, `HA`, is titrated,

`pH=pK_a+log_10{[[A^-]]/[[HA]]}`

But if `[HA]=[A^-]`, then `log_10{[[A^-]]/[[HA]]}=log_(10)1=0` and CLEARLY `pH=pK_a`. And thus the `pH` at half-equivalence `=` `pK_a`.

See this old answer.