# When titrating a weak acid should only undissociated acid molecules be considered?

Let's say we do the titration of a weak acid (acetic acid in this case) and a strong base (sodium hydroxide) represented by the following equation: $$\ce{CH3COOH + NaOH -> CH3COONa + H2O}$$

My question is that, since $\ce{CH3COOH}$ is partially dissociated, does $\ce{NaOH}$ react only with the partially dissociated part or the complete acetic acid solution?

• – Ivan Neretin Oct 13 '17 at 4:30
• Your formula is nonsense for a titration. Your base is already dissolved, and so stays sodium acetate. OH- reacts with undissociated acetic acid, nothing else. Write up reactions with reactants in the form you actually use them and you will never make such (stupid) mistakes. Insult goes to your teacher, not you, btw. – Karl Oct 13 '17 at 8:11

Acetic acid and acetate are connected by the dissociation equilibrium $(1)$:
$$\ce{CH3COOH + H2O <<=> H3O+ + CH3COO-}\tag{1}$$
Therefore, on a very minute level hydroxide reacts with neither acetic acid nor acetate but with hydronium which is produced in the dissociation of acetic acid. In practice, that still means that hydroxide will be added until $n(\ce{CH3COOH})+n(\ce{CH3COO-})=n(\ce{OH-})$. From this point onwards, there will be more hydroxide than acidic particles causing the $\mathrm{pH}$ value to rise significantly, the indicator to change colour and the student to realise that they reached (and passed) the equivalence point.