[LDB]

Hello

I need to calculate the volumes of citric acid and sodium citrate needed to make a buffer with a pH of 2. Given is that the concentration of citric acid as well as the concentration of sodium citrate is 0.1 M. After you mixed those two together you will add some water so that the total volume is 10 ml. I started with the formula of Henderson-Hasselbalch. pH = pKa + log ([conjugated base]/[acid]). I found that citric acid has 3 pKa values: 3.15; 4.77 and 6.40. I've used the first value. Now I know that the [conjugated base]/[acid] = 10^(-1.15). The problem is that I don't really know what to do next. Is this first step correct? And what's the next step.

Thank you for you knowledge.

LDB

[Borek]

Citrate buffer is a very poor choice for the pH 2.0 buffer.

What is pH of the 0.1 M citric acid solution? Will it get lower when you add a base?

[AWK]

Buffers based on citric acid start at pH ~3.

[Babcock_Hall]

The first step is correct.  The problem could be worded a little bit more clearly, but you should now write a second equation in your two unknowns, one that is based upon the total number of moles of citrate in all forms.