chemistry emergency 911?

Question

Consider an acetate buffering system consisting of acetic acid and sodium acetate where the total acetate concentration is 0.50M. What is the concentration of acetate ion (Ac-) in such a buffer at the pKa of acetic acid, ~4.75?

Answer 1

First of all the proper way to write the acetate ion is AcO-

Use the Henderson-Hasselbach equation:

pH= pKa+ log [AcO-]/[AcOH]
Thus for pH=pKa,
log [AcO-]/[AcOH]=0 =>
[AcO-]/[AcOH] =1=>
[AcO-]=[AcOH]
but total acetate= [AcO-]+[AcOH]
so total acetate= [AcO-]+[AcO-] =2[AcO-] =0.50 =>
[AcO-]=0.25 M