[MrEQ]

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⁺] / K_a.

[H⁺] for a compound with pH=2.4 is 10^-2.4. But what is K_a? 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?

[Borek]

Ka is an acid dissociation constant - different for every acid.

[MrEQ]

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?

[Borek]

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.

[MrEQ]

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?

[Borek]

Browse pKa tables, see which acids have pKa close to 2.4.