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

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Answer 1 · 2017-04-25T07:35:51

$pH=4.21$ at half equivalence...................

We use the buffer equation,

$pH=pK_a+log_10{[[A^-]]/[[HA]]}$ , which is derived [here.](https://socratic.org/questions/how-do-buffers-maintain-ph)

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.

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