Question

In the synthesis of aspirin: Why acetic anhydride is used instead of acetic acid?

Answer 1

Because the anhydride is a much stronger acylating agent than acetic acid.

Explanation:

Moreover, acetic acid is stronger as a Brönsted acid, whereas you need a Lewis acid, i.e. a positive carbon as acetic anhydride or acetyl chloride, even stronger.
Consider that if a carboxylic acid as acetic acid were capable to acylate the scarcely nucleophilic phenolic oxygen of saycilic acid, then this substrate would be able to acylate itself. In fact in the structure of salycilic acid you have a carboxylic group which is similar to the one of CH₃COOH.