Effect of dilution on weak acids pH My textbook has a question which says what would happen to the pH of a weak acid if it is diluted with water. The answer is its pH would decrease.
My teacher’s argument is that it would indeed decrease because water ionizes to hydronium ions which would increase the concentration of H+ ions and therefore, decrease the pH.
My argument is that it would increase because when the acid is diluted, the concentration of H+ ions will decrease and hence, the pH will increase. Further details in the attached photo.
My father, who is a chemist, argues that it would decrease then increase. He says that it varies upon the amount of water added and that it is not a direct relation. Instead, it would form a graph similar to the letter V.
I am honestly lost. What would exactly happen?
Whatever the nature of the acids, weak or strong, dilution of an acidic solution produces an increase of pH. The dissociation coefficient may increase, but the pH will always increase. The expression &quot;like a Y letter&quot; is pure fantasy.
Your calculations about acetic acid are correct. Just a small remark : in the second part, you say :&quot;if  you double the volume&quot;. It should be stated that this operation is made by adding pure water. In this case, the total concentration of acid is divided by 2. Surprisingly, the concentration of $\ce{H+}$ is not divided by $2$ : $\ce{[H^+]}$ = $2.99 ·10^{-3} mol L^{-1}$. So it has been divided by $√2$ (and not by $2$).
Now, let's speak about the number of moles. During the dilution, the total number of moles of acetic acid has not changed, but the total number of moles of $\ce{H^+}$ has slightly increased during the dilution, which means that the acid is more dissociated in the diluted solution.