Ka from pH of a solution

Question

The pH for 0.0715 M solution of CCl₃CO₂H is 1.40. Using a RICE table and the definition of Ka, determine the value of Ka for CCl₃CO₂H.

Answer 1

CCl₃CO₂H ======> H⁺ + CCl₃CO₂^- dissociation of the weak acid

Before writing the RICE table, let's find the [H⁺] and [CCl₃CO₂^-]

From pH, we find the [H⁺]; [H⁺] = 1x10^-1.40 = 3.98x10^-2 M = 0.0398 M. This is also the [CCl₃CO₂^-]

Now we can fill in the RICE table

CCl₃CO₂H ======> H⁺ + CCl₃CO₂^-... Reaction

0.0715.........................0............0..............Initial

0.0398.......................+0.0398...+0.0398..Change

0.0317.......................0.0398.....0.0398....Equilibrium

Ka = [H⁺][CCl₃CO₂^-] / [CCl₃CO₂H]

Ka = (0.0398)(0.0398) / (0.0317)

Ka = 0.0499 = 5.00x10^-2