When must parentheses be used in chemical formulas?
When you want to express a substance that has polyatomic ion or functional group (in organic chemistry).
There are two cases: polyatomic ions and polyatomic functional group.
 For example, Ammonium Chrolide
Ammonium Sulfate should be expressed as
It must not be
 Another example can be seen in organic chemistry.
Xylene molecule has two methyl group on the benzene ring. It is expressed as
Ethylbenzene molecule has one ethyl group on the benzene ring and its condensed formula is
Both xylene and ethylbenzene have