How do you determine the Ka of a weak acid or the Kb of a weak base?

1 Answer
anor277
Apr 4, 2017

How else but by measurement?

Explanation:

Typically, a weak acid is titrated by a strong base, or a weak base is titrated by a strong acid. In order to measure #K_a# (or more usually #-log_10K_a=pK_a#).

We use the relationship:

#pH=pK_a+log_10{[[A^-]]/[[HA]]}#

So do the measurement, i.e. the titration of weak acid by a strong base, and measure the #pH# as you add increments of the titrant. At the point of HALF EQUIVALENCE, i.e. when #[A^-]=[HA]#, we use the given equation:

#pH=pK_a+log_10{1},# i.e. #pH=pK_a#, because #log_(10)1=0#.

Satisfied?

See here for details of the buffer equation.

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