Thank you for the opportunity to help you with your question!

First we need to write the chemical reaction that occurs when the acid
dissociates. HC9H8NO3 ---> C9H8NO3- H We can write an equilibrium
expression for this reaction, products/reactants. K = [C9H8NO3-] [H ] /
[HC9H8NO3] Now, we need to think about what is happening during
dissociation. We start out with a certain concentration of the acid.
thus, when it dissociates it loses concentration. lets say it loses x
amount of concentration. so if the initial concentration of the acid is
0.120M, then at equilibrium it will be 0.120 - x. Now, when it
dissociates, the products gain concentration.

Since pKa = -log pKa

Ka = 24 * 10 -5

HC9H8NO3 = H3O C9H8NO3 0.12-x x x Ka = x 2
/(0.12-x) x 2 24 *10 -5 x - 4.56* 10 -5 =0

After solving the equation x
= 6.634*10 -3 M

Conc. of H30 =6.634*10 -3 M

Please let me know if you need any clarification. I'm always happy to answer your questions.

Jun 13th, 2015

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