How are #pK_a#, and #K_a# values for weak acids found?

1 Answer
anor277
Dec 19, 2016

#pK_a# and #K_a# values are measured not found.

Explanation:

Weak acids, #HA#, dissociate according to the following equilibrium:

#HA(aq) + H_2O(l) rightleftharpoonsH_3O^+ + A^-#

By definition, #K_a=([H_3O^+][A^-])/([HA])#

We can take #-log_10# of both sides, to give............

#pH=pK_a + log_10([[A^-]]/[[HA]])#

Several glass electrodes are available that are calibrated to read #pH#. Both weak acids and weak bases can have their dissociation constants measured by these means. Of course, given the equation, the #pK_a# of the acid, is the #pH# at the point of half-equivalence. Why?

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