[tanjinjack]

You can't mix intensive and extensive properties. Even if number of molecules grows volume grows too, these two effects cancel out. What is important is concentration. Stick to it.

But do the volume and number of molecules grow at the same rate? If that is so, concentration will not have change, and pH will have no change, but that is NOT the case.

[Borek]

Concentration will change, you are adding water, not changing amount of acid.

[tanjinjack]

Alright..

I guess I need some explanations..
What is the change in pH, why?

If pH increases, it does not follow Le Chateliers.

If it decreases, it does not follow common sense that the acid will be diluted.

 

[Borek]

I am trying for the last time. You seem to be doing some mistakes in your understanding of the very basic principles; no wonder you are misunderstanding more complicated things.

All the time you are forgetting about mass conservation - you have to deal with given amount of substance. Le Chatelier's principle tells you in which direction reaction will be shifted, but it doesn't mean that amount of substance will grow. When diluting weak acid you are shifting the equilibrium to the right, but amount of produced H⁺ is limited by the amount of acid.

Think about it this way - you stated earlier that you have no problem with understanding how the strong acid pH changes during dilution. Situation with weak acid is not much different - just there is always even a little bit less H⁺, as acid is not fully dissociated.

[tanjinjack]

Alright, I must thank everyone help's on this topic.

I have spoken to my lecturer and she accepted my explanation.
Here it is.

The equation is HA+H₂O⇌ H3O⁺ + A^-.
By Le Chatelier's, it's actually the number of mols of hydronium ion increases, not necessarily the concentration increases. This is because when adding water, the volume increases and we are not sure if the number of mols increases faster or the volume, hence the concentration is undetermined by Le Chatelier.

Then, I use the acidity constant.
K_a=[H⁺][A^-]/[HA]

As we know for sure the [HA] decreases from the equilibrium equation (number of mols decreases, volume increases), hence from the K_a, we know the [H+] also decreases.
So, we should see an increase in pH when water is added to weak acid.

Another supporting point is, when water is added into the strong acid, the acid gets diluted, and the H+ ions dissociate fully. For the weak acid, when water is added, it gets diluted as well, but it produces even lesser amount of H+, so surely it would get diluted like how the strong acid (for its being fully dissociated still get an increased pH), not to say the partially dissociated weak acid.

So, how was my explanation? Is there any mistake?

Anyway, I thank very much for your help and taking your time to read my thread.
Thanks!