Final answer:
The acidity of p-methoxybenzoic acid is weaker than benzoic acid due to resonance stabilization of the negative charge. P-chlorobenzoic acid is a stronger acid than benzoic acid because of the electron-withdrawing effect of the chlorine atom.
Step-by-step explanation:
When comparing the acidity of p-methoxybenzoic acid to benzoic acid, we need to consider the stability of their conjugate bases. In p-methoxybenzoic acid, resonance can occur due to the presence of the methoxy group (-OCH₃), which delocalizes the negative charge. This resonance stabilization makes p-methoxybenzoic acid a weaker acid than benzoic acid.
On the other hand, in p-chlorobenzoic acid, the electron-withdrawing effect of the chlorine atom (-Cl) increases the acidity. The presence of the chlorine atom causes the negative charge in the conjugate base to be more stable, making p-chlorobenzoic acid a stronger acid than benzoic acid. Resonance structures can be used to illustrate the difference in acidity between these compounds.