Why does pH = pK when pH = 7?

1 Answer
Aug 20, 2016

Answer:

#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.