(Original post by Killjoy-)
because it will enable us to find the answer. Its best to find this ratio first.
If you start with the pH you want your buffer to be, and the Ka of the weak acid, then you can find the ratio
from either of these equations:
(The second is formed from the first by taking logs to base ten.)
The ratio is dependent on the volume of NaOH added in two ways:
NaOH will react with the HA, removing some of it,
In the process it will also release A^-(aq) ions from the ionic salt NaA, formed during the neutralisation reaction.
NaOH + HA -> NaA + H2O (2)
NaA + aq -> Na^+(aq) + A^-(aq)
What follows is an explanation of the method of the poster.
So we can find the volume with a little bit of algebraic manipulation once the ratio is found by letting one of the unknown quantities be x and writing the other in terms of it.
So the poster has found the moles of acid they started with and looked at how many will remain after the NaOH has been neutralised. This will be the initial number - number that have reacted.
Calling the number of HA that have reacted x, they can deduce [A^-(aq)]=x from (2).
Also, we need not worry about the volume of solution since there's the same number of substances in the numerator and denominator of the fraction; the volumes will cancel so we may use moles directly rather than concentration.
Notice that we have assumed all the A^-(aq) formed is due to the neutralisation reaction and not the normal dissociation of the acid. This is because the acid is weak.