[sssa]

Hi guys, this it’s my first post in this Chemistry Forum. I’m a Spanish boy, and in fact I’m studying first of Biology. Well, I will be very delighted if someone knew how to solve this problem of General Chemistry. Surely that it’s nonsense…But I don’t know very much, how it’s the balance reaction, and I can not continue.

The buffer solutions of ascorbic acid / ascorbate are used in biochemical laboratorios. The value of Ka for the ascorbic acid it’s 7.9x10^(-5).  Calculate:

a) The ratio of concentrations [ascorbic acid] / [ascorbate] that forms solutions with pH= 4.
b) The ratio of concentrations [ascorbic acid] / [ascorbate] that produce solutions with pH= 4.5

Write the ascorbic acid as HA.

Thaank you, so much! And I'm waiting for yours replys  

[Borek]

You need to use Henderson-Hasselbalch equation.

[sssa]

You need to use Henderson-Hasselbalch equation.

Right, but the result (number) of log([A-]/[HA]) it's the same that the ratio (or proportion) between ascorbic acid/ascorbate solutions?

And, another question for someone more, in this problem at first ask for "form" and in the other question say "produce".. What's the diference? Or it's the same?

We can take any conclusion of the results of this two questions (together)? I'm a little losed.. but I'm a good student, and I understand explanations quickly.

Thanks Borek, and thanks everyone who wanna help me 

[Borek]

I am not sure if I understand what you mean.

HA/A^- are short forms for representing acid (HA) and conjugate base (A^-). So HA stands for ascorbic acid and A^- for ascorbate.

My bet is that form and produce mean the same - but I have no idea if you work with English text, or if it is your translation of some Spanish text, so I can be wrong.

[sssa]

Well, it's a traducction but excepting my low level of English, I think that I wrote it well. In fact, I thougth that form and produce mean the same, as well.

Anyway I think, that unfortunately you don't understood what I tried to ask you.. Of course that I knew the meanings of HA and A-, but the statement asks me for the "ratio of concentrations" of these solutions.

Before, I tried to ask you if once I use the Henderson-Hasselbalch equation, this ratio of concentrations it's the result number of the "Log." only?



So..I need only replace my pH and my Ka, and the result number it's the ratio?

[Borek]

No, the result is logarithm of the ratio.

[sssa]

No, the result is logarithm of the ratio.

Okey thank you very much!! This  it's that I thought! But the final question. There any explanation, that when I do the problem with pH=4 I get a ratio=0.79, and when I do the problem with pH=4.5 I get a ratio=2.498.

I use pKa=4.1023 as the "-log(Ka)" and the pH of the statements. And when I get the result as logaritm I do 10^x of that. It's correct I think.. so why this difference?? 

Very thanks! I'm fully understanding ^^

[Borek]

Are you asking why ratio of [HA]/[A^-] depends on pH? Simple, that's because ratio of [HA]/[A^-] depends on pH.

These are two different solutions, why do you expect them to be identical?