A 0.085 M solution of this substance is found to have a pH of 2.68. Calculate th

Question

A 0.085 M solution of this substance is found to have a pH of 2.68. Calculate the Ka for this acid. Can you please show your work? I'm trying to understand.

Explanation / Answer

First add water to the phenylacetic acid... so it would be: HC8H7O2 + H2O ---> H3O + C8H7O2 then it would look like this: HC8H7O2 + H2O ---> H3O + C8H7O2 0.085M 0 0 and since pH is given to you already, use inverse log because you use -log to find the pH. So.. it would be 10^-2.68 which gives you ~0.0021 HC8H7O2 + H2O ---> H3O + C8H7O2 0.085M 0 0 -0.0021 +0.0021 +0.0021 0.083 0.0021 0.0021 pKa= [C8H7O2] [H3O] / [HC8H7O2] [0.0021][0.0021]/[0.083] which gives you 5.31 x 10^-5

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