percent ionization, using Ka value

[Sunil]

(a) Calculate the percent ionization of 0.0075 M butanoic acid (Ka = 1.5 x 10^-5).

I did this problem, but answer didn't match the book.

I set up ice chart and everything, but I don't how to use the table feature in here to show it to you. I used the equation:

HC4H7O2 <--> H⁺ + C4H7O2-

For my equilibrium terms, I got (0.0075 - x), x, x, respectively.

For my Ka expression, I had: x² / (0.0075 - x)

Finally my quadratic equation: -x² - 1.4_E^-14x + 1.05_E^-6 = 0

x = 0.001024695 M, and percent ionization = (0.001024695 M /0.0075 M) = 13.66%

[Sunil]

Also, I have a question about the following problem too:

"Explain why a mixture of HC2H3O2 and NaC2H3O2 can act as buffer while a mixture of HCl and NaCl cannot."

My answer was that a buffered solution contains a weak conjugate acid-base pair, not strong acid like HCl. I have a feeling that it wont be enough on a AP exam. Is it correct?

[spock]

Based on the [HA] and the Ka value I'm coming up with a [H+] of 3.28E-04.

I'd check out out your quadratic formula, I think it should be:
                    x2    +  1.5x10-5 x   - 1.125E-7 = 0
That will lead the concentration of H+ above.

I think your answer about the buffers isn't bad.  You might improve it by stating that since acetic acid is a weak acid, its conjugate base is relatively strong and thus able to neutralize added acid.  HCl is a strong acid and its conjugate base is very weak and thus has little ability to neutralize any additional acid.