Using two approximations for the dissociation of a weak acid, we can write
One of these two approximations we use is that the concentration of the acid HA has not changed upon dissociation, because the dissociation of a weak acid is so small compared to the starting concentration of the acid that is negligible.
However, I was taught that this approximation is invalid when the acid is stronger, but also when the solution is very dilute.
I don't understand why this approximation wouldn't work if the solution is very dilute. I have some ideas though:
The acid concentration decreases significantly when dissolved in a lot of water, so maybe the H+ would become significant then?
Maybe the H+ from water's dissociation affect the equilibrium of the weak acid's dissociation so that the dissociation becomes significant?
Are any of these right? Could someone explain why this approximation is invalid for very dilute solutions?