[Millie!]

Hi, I need to calculate the relative quantities of 2 reagents used in a buffer solution (CH3COOH, CH3COONa) which would give a pH of 4.5.

I'm not sure where to go with this, I thought I would work backwards with the Henderson-Hasselbach equation but there is no Ka given, and I'm not sure what I would then do with the conc if I got to that stage.

I'm quite confused!

[Borek]

Take pKa from tables.

[Millie!]

So as far as I see, it would give me a pKa of 4-5 (acid RCOOH, base RCOO-), so then if I take the midpoint of the pKa,

log10([A-]/[HA]) = 4.5 - 4.5
log10([A-]/[HA]) = 0
So then, [A-]/[HA] = 1??

If they are the same concentration, wouldn't the amount of each that I would need be the same?

Sorry if this is really easy, I am struggling after having not done any chemistry in so long :/

[Borek]

No idea where did you get 4-5 (whatever it means) from, pKa for acetic acid is 4.75.

then if I take the midpoint of the pKa

No idea what you mean. But your calculations are not completely off.

[Millie!]

I took the value from my data book which gives 'approx pKa values for selected organic compounds', and then I took the midpoint of the range it gave me. Is there a good resource online for pKa tables?

If using 4.75 I would get,
log10([A-]/[HA]) = -0.25
[A-]/[HA] = 10^-0.25
[A-]/[HA] = 0.56

So that would mean I have worked out that the conc of CH3COOH is approx twice that of CH3COO-, so how do I then incorporate CH3COONa into the equation in order to work out the relative quantity? Would I do the same again with the pKa and then compare the ratios?

[Borek]

and then I took the midpoint of the range it gave me.

There was _range_ given for acetic acid pKa? This is ridiculous.

Is there a good resource online for pKa tables?

Try for example http://evans.harvard.edu/pdf/evans_pKa_table.pdf

So that would mean I have worked out that the conc of CH3COOH is approx twice that of CH3COO-, so how do I then incorporate CH3COONa into the equation in order to work out the relative quantity? Would I do the same again with the pKa and then compare the ratios?

CH₃COONa is a source of CH₃COO^-, not sure what you are trying to do. How much CH₃COONa you dissolve that much CH₃COO^- you will have (that's not entirely true, but that's a very good approximation).

Now, question is not complete - that is, it doesn't tell you what concentration of the buffer you need. You may now assume - for example - concentration of acetic acid should be 1M (or 0.1M, or 0.01M - doesn't matter much) and calculate concentration of acetate needed. Or you can assume concentration of acetate (again - it can be whatever you want, 1M, 0.1M, 0.01M) and calculate amount of acetic acid. Or - which is the best approach, although requires more work - you can assume that you want sum of concentrations of acetic acid and acetate to be known and go from there (hint: you will have to solve simultaneous equations). Last approach is the best, as you will be quite often said to prepare buffer solution of a given concentration - that means sum of acid and conjugate base concentration.

[Millie!]

Thanks for the link.

Okay so if I work out those concentrations won't that just give me the amounts of CH3COOH and CH3COO- I would need for the buffer solution?

I need to work out the amounts of both CH3COOH and CH3COONa relative to each other for the solution, so I suppose the ratio.

Would the concentration of CH3COONa be the same as that of CH3COO- from the other reaction?

I am presuming my buffer solution would consist of these 2 reactions:
CH3COOH + H2O <--> CH3COO- + H3O+
CH3COONa <--> CH2COO- + Na+