Which Ka value to use in the Henderson-Hasselbalch equation for buffer problems

[STAR3URY]

Very Urgent: Need Help With Buffers!

I have a test coming up on Tuesday and I have no idea how to do buffers, I don't understand what Value of the Ka we are suppose to use from the chart. I get the equation a little bit: [H+]= Ka x moles HB/moles of B-

I am very lost please help!

 

[symbolipoint]

What? Your problem description and question are poor. What are your knowns and what are you unknowns?

 

[Kushal]

ok... i'll try to help you out a bit...

you should know that buffers resist drastic change in pH.

basically, the H+ ion concentration in a buffer solution is:

Ka * [acid] / [salt]

or for an alkaline buffer

Kb * [alkali] / [salt][acid] would mean the concentration of the acid in mol/dm3

if you use ethanoic acid and sodium ethanoate as the buffer, then H+ concentration is

dissociation constant of ethanoic acid * concentration of ethanoic acid / concentration of sodium ethanoate

the question will usually ask you about the pH of the buffer. to get the pH:

pH = -lg [H+]