Why Doesn't Adding Water Change the pH of Acid During Titration?

[alingy1]

Homework Statement

Ok, so, I've been thinking about this for a while now.

When I was doing titration back in high school, we had to put water (so that the solution would have a larger volume to work with) in the acid in which we were going to pour base from a buret.

Now, my question is this:

Why doesn't the water change the pH of the acid? You add more water which dilutes the concentration of H+. So, then, there is the equilibrium formula:

K=[H+][A-]/[HA]

Since you dilute the solution (let's assume by doubling the volume of the solution), the K changes:
[0.5H+][0.5A-]/[0.5HA]

So, equilibrium constant is not the same anymore. Shouldn't this cause changes in the concentration of the acid and thereby false the titration?

I feel there is something that doesn't click here.

 

[Borek]

Why doesn't the water change the pH of the acid?

It does.

You add more water which dilutes the concentration of H+.

It also dilutes all other things present in the solution.

Since you dilute the solution (let's assume by doubling the volume of the solution), the K changes:
[0.5H+][0.5A-]/[0.5HA]

No, K doesn't change. Equilibrium shifts till K has exactly the same value it had before.

So, equilibrium constant is not the same anymore. Shouldn't this cause changes in the concentration of the acid and thereby false the titration?

K didn't change, but even if it would, it would not change the titration result. Amount of acid (note: amount, not concentration!) is calculated from the neutralization stoichiometry, not from pH.