April 17, 2021, 10:46:24 PM
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Topic: Finding Ka and pKa of unknown acid in a buffer solution?  (Read 204 times)

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Offline Caramelcorn14

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Finding Ka and pKa of unknown acid in a buffer solution?
« on: March 31, 2021, 06:17:27 PM »
I'm working on a lab on acids, bases, and buffers, and I need to find the Ka and pKa of an unknown acid in a buffer. I added 1 gram of the unknown acid (solid) into 100.0 mL of DI water, then transferred half of that solution (50 mL) into a flask and added 2 drops of phenolphthalein to the flask. I titrated it to the endpoint with 0.5 M NaOH. I was told to disregard the volume of NaOH used. Then I combined the neutralized half with the other half, and that's my buffer.

The pH of the buffer is 5.04. I know that pKa= -log(Ka), and [H3O+] for this buffer is 9.1201E-6. I use that in the equation for Ka, Ka=[H3O+][A−]/[HA]. The issue is I don't know the concentration of the unknown acid, and how do I find the concentration of NaOH in the buffer if I don't know the volume added during the titration? Once I get those values I'll be good to go, but this is really stumping me.

Offline Babcock_Hall

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Re: Finding Ka and pKa of unknown acid in a buffer solution?
« Reply #1 on: March 31, 2021, 06:22:13 PM »
There is no NaOH to speak of in the buffer.  You have enough information in front of you to answer the question, but you have to think a little outside of the box.

Offline Borek

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Re: Finding Ka and pKa of unknown acid in a buffer solution?
« Reply #2 on: April 01, 2021, 03:10:37 AM »
Buffer is a mixture of acid and its conjugate base. NaOH - while a base - is not the base you are looking for.

Hint: you don't need concentrations of the acid and its conjugate base, you need ratio of concentrations. This can be calculated without knowing concentrations themselves.
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