[elad]

Hi,

Today, I had a test and there was a question which I didn't succeed to answer:

You should prepare a watery buffer in volume of 1 liter, with pH=8.41, and a total concentration of 0.38M.
You can use the salt pridinium chloride C5H6NCl (Mw=115.56gr/mol, you can presume the salt is fully soluble), HCl 0.2M solution, NaOH 0.2M solution.
pKa(pyridinium chloride)=8.75
Is it possible to prepare such a buffer and how?

The answers were:
a. 43.9 gr of the salt, 600 ml NaOH, 400ml water.
b. 43.9 gr of the salt, 600 ml HCl, 400 ml water.
c. It is not possible to prepare such a buffer.
d. 43.9 gr salt, 300ml NaOH, 700 ml water.
e. 43.9 gr salt, 300ml HCl, 700 ml water.

I didn't succeed answering this question because I didn't know how the salt is dissolved and how it becomes an acid.

Thanks.

[Borek]

Then you should brush on the Bronsted-Lowry theory of acids and bases, and look for the Henderson-Hasselbalch equation (which is nothing else, but rearranged dissociation constant definition).

[AWK]

Pka is for pyridinium cation, not for salt. Treat it in the same way as NH₄⁺, but with a different value of pKa.