Hi, am just a bit unsure about this area of the topic:

 

When a weak acid dissociates into a proton and anion ...

 

HA <==> H+ + A-

 

Does the concentration of hydrogen ions (H+) always equal the concentration of the anion (A-) here?

 

Also I have found the formula

 

Total concentration = Concentration of ionised + concentration of undissociated.

 

To use this formula would I take the "concentration of ionised" to be just

[A-] or would it be [A-] + [H+]? I was thinking it would be just [A-]. Though am unsure of the explaination.

 

Clearly for "concentration of undissociated" this would be [HA] value.

 

Any help with this confusion would be greatly appreciated!

 

Many Thanks!

The concentration of protons only equals the concentration of anions if the solution had no protons to begin with, or if the original concentration of protons in solution was negligibly small compared to the concentration of protons produced by ionization. It might not be the concentration of anions if you had a pH buffer in solution as well.

 

Concentration of ionized is just referring to [A-]. That keeps the total concentration constant no matter how much disassociates.