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

1 Answer
Apr 25, 2017

Answer:

#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.