Question

When a weak acid, for which `K_a=6,2xx10^-5` is titrated with a strong base, what is the `pH` at half-equivalence?

Answer 1

`pH=4.21` at half equivalence...................

Explanation:

We use the buffer equation,

`pH=pK_a+log_10{[[A^-]]/[[HA]]}`, which is derived here.

But at half equivalence, `[HA]=[A^-]` BY DEFINITON, and since `log_10([[A^-]]/[[HA]])=log_10(1)=0`, then `pH=pK_a`

`K_a=6.2xx10^-5`, and this `pK_a=-log_10(6.2xx10^-5)=4.21`.

And thus `pH=4.21` at the point of half-equivalence. Note that the concentration of the weak acid is irrelevant.