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Topic: Preparing a 500ml buffer with a set pH  (Read 245 times)

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Offline MrEQ

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Preparing a 500ml buffer with a set pH
« on: December 05, 2021, 09:51:41 AM »
Hey,

I have a problem I'm struggling to solve. There's something fundamental I'm missing here.

The problem asks us to describe how to create a 500ml buffer with a pH of 2.4.

My first problem is that I don't know which compounds to use. I understand that there's some equation we can use whereby the ratio of the concentrations of the two compounds, [Compound1]/[Compound2] = [H+]/Ka.

An example in my book uses acetic acid and sodiumacetate, and demonstrates using HC2H3O2 and C2H3O2-.

If I want to use the same substances, could I do the following?

[HC2H3O2] / [C2H3O2-] = [H+] / Ka.

[H+] for a compound with pH=2.4 is 10-2.4. But what is Ka? Does it vary? The example uses 1.8*10-5, but I'm not sure if this is constant.

Am I on the right track here?

Offline Borek

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Re: Preparing a 500ml buffer with a set pH
« Reply #1 on: December 05, 2021, 10:05:34 AM »
Ka is an acid dissociation constant - different for every acid.
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Offline MrEQ

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Re: Preparing a 500ml buffer with a set pH
« Reply #2 on: December 05, 2021, 10:12:29 AM »
My attempt involves Ka = [H+][C2H3O2-] / [HC2H3O2] = 1.8 * 10-5.

If I use this, I get the ratio 10-2.4 / 1.8 * 10-5 ≈ 221, but that seems like an unlikely ratio, right?

Offline Borek

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Re: Preparing a 500ml buffer with a set pH
« Reply #3 on: December 05, 2021, 10:29:50 AM »
No, the calculated ratio seems to be about right. But for a buffer you need an acid with pKa as close to the buffer pH as possible - that means at pH=2.4 acetic acid is out of the question.
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Offline MrEQ

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Re: Preparing a 500ml buffer with a set pH
« Reply #4 on: December 05, 2021, 10:52:00 AM »
Thanks for the feedback! Good to know the calculation was somewhat right, albeit a dead end.

I guess I have to choose my compounds better. Any tips?

Offline Borek

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Re: Preparing a 500ml buffer with a set pH
« Reply #5 on: December 05, 2021, 11:27:53 AM »
Browse pKa tables, see which acids have pKa close to 2.4.
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