How do you write anhydride formula?

Hint: An anhydride is a group of organic compounds, basically anhydride means the compound is without water. It is formed when two carboxylic acids react with each other, then a compound is formed after the removal of a water molecule. The anhydride functional group is present in the middle of the chain.

Complete answer:
An anhydride is a group of organic compounds, basically, anhydride means the compound is without water. An anhydride is a functional group whose formula is $-COOOC-$. In this formula, both the carbon atoms are joined with the oxygen atom by the double bond and there is one oxygen atom is joined between both the carbon atoms.
The anhydride compounds are formed when two carboxylic acids react with each other. The product is formed by the elimination of a water molecule. The reaction is given below:

As we can see that this functional group cannot be present at the terminal carbon atom, hence, this functional group is present at the middle of the carbon chain. The formula can be written as ${{(RC(O))}_{2}}O$.
The R is the alkyl group, it can be the same, which will form symmetrical anhydride. For example, if the value of R is methyl ($C{{H}_{3}}$) then the compound is acetic anhydride. The formula is ${{(C{{H}_{3}}CO)}_{2}}O$. The open formula is:


The R is the alkyl group, it can be different, which will form unsymmetrical anhydride. For example, if the value of R is methyl ($C{{H}_{3}}$) and ethyl (${{C}_{2}}{{H}_{5}}$) then the compound is acetic anhydride. The formula is $C{{H}_{3}}-C(O)-O-(O)C-{{C}_{2}}{{H}_{5}}$. The open formula is:


Note:
The Friedel-craft acetylation is an organic reaction in which many aliphatic and aromatic acids are treated with an acid anhydride. The acetate esters are majorly formed from acetic anhydride.