Acetic acid on dehydration produce acetic anhydride. $ {{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}} $ is a dehydrating agent it dehydrate $ \text{C}{{\text{H}}_{3}}COOH $ to anhydride $ \underset{\text{Acetic}\,\text{acid}}{\mathop{2C{{H}_{3}}COOH}}\,\xrightarrow[(-{{H}_{2}}O)]{{{P}_{2}}{{O}_{5}}}\underset{\text{Acetic}\,\text{anhydride}}{\mathop{{{(C{{H}_{3}}CO)}_{2}}O}}\, $