Question about relevance

[t35t]

Hi,

I was looking through a book, and I found this equation:
pH=pKa +log[proton acceptor]/proton donor

derived from Ka=[H+][A-]/[HA]

with regards to the titration curve for weak acids, buffers etc where -log[H+] and -logKa has been substituted for pKa?

I know it looks like a silly question, but honestly, after the ridiculousness of unit 3 exams, and the KBT exam, i honestly thought that this equation seemed too important to forget, and I had a slight feeling that it will eventually appear down the track where I might be asked to calculate say, the ratio of the concentration of acetate and acetic acid of a system (for a buffer) required for a certain pH.

So, for people who know, I was wondering if this wouldever come in handy? If it's worth looking into more, or if I should just ignore it?

[Jenny_2108]

I might be asked to calculate say, the ratio of the concentration of acetate and acetic acid of a system (for a buffer) required for a certain pH.
So, for people who know, I was wondering if this wouldever come in handy? If it's worth looking into more, or if I should just ignore it?

If the question gives you the pH of solution, you can calculate [H+]= 10^-pH

From the databooklet, you can get Ka of acetic acid=1.7*10^-5

So the ratio of the concentration of acetate and acetic acid= Ka/[H+]= 1.7*10^-5/10^-pH

[t35t]

o thanks!!!

I was wondering then, if you could help me with this question:
"calculate the ratio of the concentration of acetate and acetic acid required in a buffer system of pH 5.30"

[Jenny_2108]

o thanks!!!

I was wondering then, if you could help me with this question:
"calculate the ratio of the concentration of acetate and acetic acid required in a buffer system of pH 5.30"

pH=5.30 => [H+]= 10^-5.30

Ratio= 1.7*10^-5/10^-5.30= 3.39

[t35t]

Thanks again for your help!
Sorry for sounding silly, but...  I'm wondering about the set up of the ratio/arrangement-
ratio= ka/[H+]-> so, we use the Ka of acetic acid over the extent of it's ionisation to acetate [H+] in the mixture?

[Jenny_2108]

Actually the formula of the ratio is derived from:

Ka= [CH3COOH-] [H+]/ [CH3COOH]  => [CH3COO-]/[CH3COOH]=Ka/[H+]