Final answer:
The formula for the acetate ion is C2H3O2− or CH3COO−. In compounds, it is represented as is or in parentheses with a subscript if more than one ion is needed.
Step-by-step explanation:
The formula for the acetate ion is C2H3O2− or alternatively represented as CH3COO−. When writing formulas for ionic compounds containing polyatomic ions like acetate, it's necessary to preserve the polyatomic ion as a unit. If there is only one acetate ion needed to balance the charge with a cation, you simply write its formula once without parentheses. However, if more than one acetate ion is needed, you would use parentheses and a subscript to indicate the number of ions (e.g., Mg(C2H3O2)2 for magnesium acetate).
Remember, when writing the final formula for compounds containing polyatomic ions, all subscripts that are 1 are omitted, and parentheses are only used when there is more than one of the same polyatomic ion. So, in our previous magnesium acetate example, since there are two acetate ions, parentheses are used around the acetate formula with a subscript 2 outside.